I am probably going to go with the (New? I never had one but it has a bigger VC library) 3DSXL because it has one of the the biggest libraries with access to NDS and some of the very best games (or ports /remakes of the best) on any Nintendo console. I love the amount of Zelda and strategy games I have access to, the Pokemon Mystery Dungeon series, Shin Megami tensei: Strange Journey Redux, and I have only touched the surface of platformers and anything else it has to offer. Right now it's easy to explore the backlog cheap since it's not retro (unless it's an obscure game with a huge cult following like DS game Sola-to-Robo, but I haven't seen anything else terribly expensive.) I have an ant-size old 3DS right now because it fits better in the pocket but I enjoy that bigger screen far more.

The GameBoy Advance SP isn't too far off in that regard for me, since it can play the entire backlog of GB games though a couple become very backwards (Kirby Tilt & Tumble) and that built-in backlight made playing anywhere, any time comfortable. It also has a very long battery life. I had so much fun with Final Fantasy Tactics Advance, gens 1 & 2 of Pokemon, Pokemon TCG, the GB Kirbies, and the Zelda: Oracle games, that one weird DBZ game where the combat was with cards and The Legacy of Goku, cheesy co-releases to their superior console versions like Croc, and the ports of legendary NES and SNES titles so I could have them outside of the living room - Final Fantasy I & II was a perfect remake and I had endless fun with the randomized post-game dungeons of the former and the skill-leveling system of the latter.

If handheld consoles don't count, I am torn between SNES for its huge library chock full of good platformers, JRPGs and racing games, or N64 for the few fantastic and unforgettable titles plus the best controller Nintendo ever designed in my opinion.