reign.tumblr.com was definitely my favourite user, too bad it got banned. Whats also funny is that I hated Tumblr but just kept it because of the user. I also had 2 letter IGs about 6 months ago that I really regret selling.
@homie I used to own @tease. I bought it for $120 off Madison (wayne) on here and made a huge profit by selling it for $250 to Codeine. Was a great twitter.
The best username I have ever owned was fucker. I owned the kik: fucker for four days and then got scammed by Kalvin. He can vouch that this happened. Good thread idea btw.
To answer this question though, I never really got too involved with OGs (well, gamertags yes, but sms no.) so I haven't really owned much of importance, except @Tease, the kik J., and some other stuff.
The names that I really like seeing would have to include Doomsday, Maniac, Navajo, and Menace.