My 3 favourites are the GBA, the SNES and the PCE.
The SNES was not only my childhood passion, it still is my passion to this day! There's just so much good stuff on the system that I'm still discovering new things to play and love on it! I also find that so many of its games have aged beautifully, with 2D graphics that look so good that you could release a SNES game today and it wouldn't look any worse than it did back in the day, and the sound is the perfect middle-ground between chiptunes and real music, it's still limited enough that it sounds like chiptunes, but it also sounds more advanced than its predecessors and competitors! Love it!
The GBA is like a handheld SNES on-steroids! It can do practically everything the SNES can, and then some, but without any slow-down, and you can play it anywhere you want without having to worry about your battery dying so soon! cough3DScoughSwitch
As if playing excellent conversions of some of my favourite SNES games anywhere I want isn't already good enough, the system is chock-full of original titles that are just as good as, if not better than some of the best 3rd and 4th gen classics! I've also made it no secret that I LOVE GBA sound, I know a lot of people don't, but I can't help but to find its unique mix of bit-crushed samples with GB chiptunes to be absolutely delightful! I like to say that it was the last game system to have its own "sound identity" because of that, but I guess to most people that's for the better...
The PCE is one I've discovered later, (Thanks Wii!) but it's blown me away with the sheer quality of so many of its titles! I'm a big fan of games that push hardware limits, and the PCE is the perfect middle-ground to have limits being pushed left and right while still managing to make games that look, sound and play great! Because of that, I like to think of the PCE as the most impressive game system that I've played... And of course, I can't not mention the sound! The CD audio was good and all, but the chiptunes are where it's at! They're kinda like NES and GB tunes, but taken to the next level! Having the ability to make your own waveforms, as well as more sound channels not only opens so many possibilities, but it's also MUCH easier to work with and get the sounds that you want than, say, the FM synth you get on the Mega Drive... I'm proud to be a dedicated PCE chiptuner ;3
Honourable mentions: Wii, 3DS, Mega Drive, X68000, Game Boy/Colour, NES, PSP.
