I have them all, or at least I THINK I do. I have all of the editions of D&D, from the OCE and it's Supplements, B/X, B/E/C/M/I, AD&D, AD&D 2e (and those later editions) and more clones than you can shake a stick at.
When I returned to gaming via Fantasy Grounds 2, I played Castles & Crusades. Moving to G+ Hangouts, it was D&D Next Beta which quickly became me running a game of ACKS for the group. We moved from that to playtesting some Ambition & Avarice and now I'm running a AD&D 1e / OSRIC campaign.
So, although I switch around my game systems, it's generally only one at a time (I did run some DCC RPG one shots over the summer / fall)
I guess I'm a "one system at a time type of DM / Player" for the most part.
Do you play in / run different OSR systems at the same time? Any particular reason to do so? Are you a "Serial System Changer" like me? ;)