I'm currently using Regulus 2, because it is the best compromise I can find among all of the themes that are currently available. For anybody who designs themes, or chooses themes to add to the WP.com options, I'd like to detail what I would consider my Ideal Theme:
* Tabbed Pages (Regulus 2 has it)
* No photos in the theme (Regulus 2 forces me to choose a photo, the moon is the least obtrusive -- I really wish I could just upload a photo of my choosing!)
* Clean, unobtrusive (and hopefully customizable) color scheme (Reg. 2 has it)
* Calendar to navigate recent posts (Should be a widget) (Reg. 2 has it)
* "Filed under" category links for each post (Reg. 2 doesn't have it)
* Widget support! (Reg. 2 doesn't have it!)
If a theme meeting these criteria became available, I would switch immediately!