I suppose it depends on how much I can bend the rules...

If I'm allowed to use the console only 'as-is', then probably the Nintendo DS. This gives me DS games (which are great), but also GBA games as well (though you'll miss out on GBC/GB games, which is a bit sad); this also nets you a smattering of NES/SNES ports to boot, so that's nice. But most importantly, it gets me Chrono Trigger and a bunch of my favorite Castlevania games all in one place (sad that SotN doesn't get here, but...)

If I'm allowed to use the console with no hold's barred, then Playstation Vita. Mod that little sucker and you've got access to a ton of stuff... PSV games obviously, but emulated PS1, PSP, GBA, GBC, GB, NES, SNES, and Genesis also (and maybe more, I don't think I've tried any others though).