Physically, my libraries are pretty small. My largest is, oddly enough, my PC one.
...Well, retrospectively it makes sense. After all, you can't sell PC games back to Gamestop (which, back in the day, was how I got the money to buy more games), and I've been a big Sims fan since early 2000s. As a result, while I've owned many, many games (and, if we were talking total collection, sold or otherwise, my PS2 would easily be king), thanks to my lack of income as a child, I would often sell my games back to Gamestop for their terrible return rates and get more games. But a number of years ago I stopped selling my games because I actually like collecting, and I hate selling a game only to regret it and probably buy it back later...If I can, but then there are games like the .Hack series. I was shocked when a friend told me that they were not only rare now, but they sold for upwards of $100. And I'd already sold mine.
So yeah, PC's my largest library physically, followed by the PS2 and the regrettably the Xbox 360, as much as it pains me to say. 3DS and then N64 after that. Digitally, PC would still be king, 3DS would be second, and PS3/Vita would be third. Since I buy so many games digitally nowadays, it really doesn't surprise me that PS2 and PC are my largest phys libraries, especially because there are times when I really just don't want to buy from Amazon, Gamstop in general I just can't stand anymore, and I'm not much a fan of buying games from other, non-gaming retail stores unless I find a really good price or rare gems.
Ugh...all of this reminiscing is making me remember my lost GBA library ;_;