I think it's unbalanced to lump DS and 3DS together while PSP and Vita together. IMO, PSP has the better library of interesting RPG (and other genres) compared to DS while 3DS certainly has better library compared to Vita, at least the localized ones because everyone hates Vita.
If you ask me which has better selection of interesting RPGs (for me), it's gotta be PSP > DS > 3DS > Vita. Heretic, I know, but I like the other genre more on DS.
Listing all games is kinda overkill though, so I'll list some over the top of my head.
PSPYs Seven obvsly, any other than this in the top spot is heresy. I would also list down Chronicles, Oath in Felghana and Ark of Napishtim, but I think those are better played on Steam. Though the Steam version lacks the fabulous Chester voices.
Gurumin: A Monstrous Adventure is also available on Steam, but well, I don't think it aged greatly on PC.
Nayuta no Kiseki, a.k.a. Zwei 3. It's an import, but it's a stage oriented Action-RPG, you don't need much knowledge on Japanese to play. It's very pretty, it plays slower than Ys but it's not necessarily a bad thing because the boss level design is absolutely creative (compared to Ys Memories of Celceta), and the soundtracks are beautiful. The boss theme is not really as pumping as Ys' but its field theme is great enough. And the final boss theme, whee.
Hexyz Force. A standard chibi turn-based jRPG with a rather non-standard mechanics. The game has two protagonists to play as, with them crossing paths sometimes during the standard gameplay and they band together in the final arc. Really fun, and if you're like me and felt that most RPG nowadays doesn't emphasize too much on the logistic (item/weapon) management, then you're gonna love this game. Really, the game is severely underhyped when it's actually one of the better turn-based RPG I've ever played in recent years.
Kingdom Hearts: Birth by Sleep Final Mix, easily the best of the command deck KH.
ZHP: Unlosing Ranger Vs Darkdeath Evilman. It's a roguelike-RPG hybrid. Goddamn, for a parody game, the plot can be really heartwrenching at times. I'll just
link the trailer so you can judge it yourself.
Patapon 3. Screw it, yes, it's a rhythm game, but you have 4 party members, you can change and level up classes, learn new skills; if that's not RPG, I don't know what is. I honestly like Patapon 2 better than 3, but that's only because seeing tons of spears and arrows flying around is so satisfying. The soundtracks are also really catchy; no one hates a bunch of kids screaming around in a playful but melodic manner.
ClaDun. And Cladun X2. There are several features that really appealed to me in this series, not counting the whole create your own sprite thing. I mean, it's cool enough that you can draw your own character and weapon, but that's not the only cool thing. For starters, I absolutely adore procedurally generated content from a designer standpoint. I know a lot of people think of it as a lazy way out for game designer to not have to design dungeon, but procedurally generated content is an efficient way to increase replayability with very small cost. They only need to specify certain properties beforehand and let the dungeon be populated randomly; it eliminates the need for bigger file size to store pre-bake dungeon design and let them do the whole "no two dungeon is the same!" shenanigan. Secondly, Magic Circle is honestly the best mechanic a dungeon crawler can ever come up with. It's an interesting take on the party system for a one-man dungeon crawler, making the party members a literal meatshield that protects player from the direction of attack. Of course, they're not just simply meatshield, they also provide really interesting, customizable boost to the main character as long as they are alive.
Final Fantasy Tactics: War of the Lions. You could probably get the mobile version since I heard they fixed the framerate issue.
Wild Arms XF is also another Tactic-style game. But they have hexagon instead of square like the standard Tactics game! That's awesome.
Tales of the World: Radiant Mythology. Also 2 and 3, but those aren't localized, but PSP is region free anyway. Everyone seems to hate the Descender (mute self-insert character), but I actually like that part the most in Radiant Mythology games.
The Legend of Heroes: Trails in the Sky, though you should rather get the Steam version.
Final Fantasy Anniversary Edition is basically FF1 and 2 remake, but with really gorgeous sprite and reworked gameplay I think? I'm pretty sure spells use MP instead of limited number of use per spell tier.
Dissidia Duodecim, well, it's more of a brawler than RPG, but it still has RPG-ish mechanics and even world map in the sequel. Just get the sequel than the first game because everything in first game is in the second. They kinda nerfed Dodge Cancelling in the second game though.
Adventures to Go. This game is pretty whack. Standard dungeon exploring turn based RPG with very standard setting. Guy wants to be adventurer, guy enlist in adventurer guild, and shit. Still fun timewaster. The fact that I remember it now is testifying enough of the fun I had.
Brave Story: New Traveler. One of the earliest RPG on the system. Still fun and pretty.
Persona 3 Portable because it's Persona. Really optimized for mobile platform.
Honorable mention:
Half Minute Hero because it's more puzzle game than RPG for me. But it's an RPG parody. HMH Second Coming is more of an RPG though.
Unhonorable mention:
Crisis Core FF7. I hate it, but everyone else seems to like it. Play it yourself I guess.
DSMegaman Starforce. And 2 and 3. One of the main reason I bought DS in the first place. It's a little lackluster compared to Battle Network, but is still great nonetheless. As with Battle Network, you have a deck of Battle Cards to customize with that serves as your main damage dealer in a fight, which you can get by defeating viruses quickly in battle. See the synergy of gameplay there? You have to collect good cards to get good cards. The game has no levels so grinding is only done to get multiple of cards that you want to use for your battle, so you can focus your effort in getting better steadily. Most of the cards have hidden properties that you have to experiment with to know which combination would yield awesome result. Bosses are really fun if you exploit the weaknesses and use seemingly worthless but extremely potent cards. Like this one card - Earth Thunder, it summons electric balls that paralyzes from holes and only from holes in the battlefield. So what do you do with it?
Break open every panels on the field and spam! Or you know that Fire does double damage on grass panel and Break does double damage on frozen opponent, what do you do with a card that has both properties?
Freeze them on grass panel and quadruple damage them! Caution headphones user.
Pokemon Black 2 and pretty much the other Pokemon games I guess. Diamond/Pearl/Platinum/Heart Gold/Soul Silver/Black/White/White 2. Only reason I chose Black 2 (or White 2) to put here is because of Pokemon World Tournament. Specifically monotype tournament. Damn, I really love monotype.
Digimon World Dawn/Dusk. The testament for this game addictiveness is during my first year of college. Me and my friends camped 7 days straight in the same position in my room, really devoted.in digivolving/fusing digimons. Me on the couch, a friend on my bed, another on the floor. We only stopped for food and sleep, and yeah we skipped class. Another friend.of mine can just shake his head with pitying gaze whenever he stops by.
Final Fantasy Crystal Chronicles Ring of Fates. I like the idea of Action FF. Nuff said.
The World Ends with You. Quirky mechanics, awesome collectible pins that changes the gameplay, pretty sprite based game, and a rather confusing but engaging storyline. Good game.
Kingdom Hearts 358/2 Days. Pretty ugly, but it's the closest to KH2 that we had for the spinoffs. The tetris customization panel is actually pretty fun to play with.
Final Fantasy Tactics A2 As much as I dislike the kiddifying of FFT premise on Nintendo handheld, the game is rather solid and still fun. I had fun playing the tons of sidequests that I just somehow realized I haven't actually progressed much in the storyline 20 hours in.
SMT: Devil Survivor and Devil Survivor 2. But mostly Devil Survivor because the survivor part is more emphasized in the first game. You literally have no idea what's going on and has to camp outside and scavenge for food while in 2, you pretty much gets the support of some really rich organizations right from the first day iirc? That's downplaying the crisis factor.
Golden Sun Dark Dawn I guess?
3DSPokemon Alpha Sapphire, and X/Y/OR too I guess. It's pretty much the system seller, so it's a no brainer.
Bravely Default is the old school RPG that is not at all old school. Pretty graphics, really good gameplay and stuff. There's option to off random encounter or decrease/increase the amount and automated battle, that's really great if you want to grind or simply play the story.
Kingdom Hearts Dream Drop Distance simply because Flow Motion is an interesting mechanic. Dream Eater too, if only they aren't unnecessarily colorful.
Fire Emblem Awakening. I don't like the older FE much, but I really like this one. Yeah, I'm pretty much the filthy casual fan with hard on for waifu simulator that is plaguing the FE community nowadays.
SMT IV because SMT. I dunno, any RPG list is not complete without SMT somehow.
VitaYs Memories of Celceta, because none shall ever take the top spot than Ys.
Tales of Hearts R duh, this is Tales forum after all. I may not like certain parts of Hearts R, but I love Tales series in general after all.
Persona 4 Golden is Persona. Self-explanatory.
SAO Lost Song I guess? Pretty lacking in content though.
Sorcery Saga: Curse of the Great Curry God ROGUELIKE! Or Roguelite? Anyway, the title sounds dumb, but if there's anything ZHP taught me, dumb sounding title has the best gameplay. And yeah, the game is fun as hell.
Demon Gaze. Well, tbh, I don't really like first person dungeon crawler with static image in battle, and I probably won't buy this game if it's not because of the really cheap price I get. But after trying it, it grows on me and I had a blast playing it to completion. Really hard and requires a rather intensive planning to play.