I'm a collector and all around RPG lover. I have extensive knowledge of and experience playing nearly every edition of D&D and many non-D&D RPGs. I consider myself an expert on D&D as a result.
I have NEVER met anyone who owns more RPG stuff than me overall. Though I have met a few (very few) people who own more D&D stuff than me, especially if you count third party d20 stuff. I pretty much just stuck with WotC for D&D with a few exceptions for special cases like Ptolus.
I own a lot of 1e books, and TONS of 2e books and boxed sets, filling several boxes in my garage.
I own virtually every 3e hardbound book WotC ever released, plus a selection of the best 3rd party products like Ptolus, Tome of Horrors and so on.
I own the best non-fantasy d20 stuff also. I own ALL 1e/2e Spycraft books ever published, ALL M&M books, ALL SW d20 (original, revised, and Saga edition) etc.
I own every 4e book WotC has released to date.
I also own an extensive RPG collection outside of D&D and d20 including some rare stuff like Mekton Z, and the WW Street Fighter RPG. I own virtually every HERO 5th Ed book, some Exalted, lots of WEG SW, etc. At one point, several years ago, I also owned first edition copies of EVERY single Palladium gamebook published from about 1989 to the year 2000. ALL Rifts, Robotech, TMNT, HU, N&S, BtS, etc. Over the years I have sold my Palladium stuff, though. One of the few times I actually got rid of stuff.