Most: PS2. This title has so many weird and interesting titles, it was just before the introduction of achievements and the death of things like gameshark, so experiences were still pure. It may not have aged well, but it aged better than PS1. And it was during my informative years, so I'm largely interested in checking out the things I missed. Also, the controller doesn't suck. I also have a lot of SNES, N64, PS1, and 360 games as those were my platforms of choice during their respective generations (N64 and PS1 co-exist because N64 has a small library and relatively few multi-platform titles.)
Least: I have a few platforms with only one or two titles, they are the least because they are obscure and I only have titles because I just sort of stumbled into having a single game or two for free or next to nothing. In terms of platforms that I actually have and have what I would call a collection for... I'd say Sega Master System. I only have 27 games for it and I've bought a few games for it in recent years. I have a lot of other platforms with fewer games, but I either don't really consider it a collection per se or it's just obscure or has a small library or quite frankly I don't really care about it. Like my Arcadia 2001 collection, it's like 7 games, but the total library is like 20 games.