My top three:
1. Ultimate droid- There is a reason why people mention this rom more often than others. It's awesome!!!!
I just added a theme (Reds) over it yesterday but I was completely happy without one because of how nice U.D. looks without it. You can't download U.D. 1.0.0 from Rom Manager but you can use the link above or go to droidforums.net and get it there.
2. ShadowRom- This to me is the next step down as far as having less options to work with but until I tried U.D. it was my favorite. You can find it in Rom Manager.
3. ChevyNo1 or Cyanogenmod- O.K. that makes four but I couldn't choose between these two for my third place slot. They are both great. Both of these can be found in Rom Manager.
I have been able to try quite a few roms and themes out and all of them worked. I came across some hiccups with a couple roms as follows:
1. If you get force closes on apps and market then go to rom manager and run permissions.
2. If you get force close on gmail then try re-wyping and reinstalling the rom and theme.
3. If you get the notification that your gmail won't sync then (while in the gmai app) open the menu>more>help>scroll down to troubleshooting>click problems with sync and go on from there.
4. If you install a rom and get stuck in a bootloop then go back to your backup>start your phone up again and try installing a different kernel.
All of this is off of the top of my head so guys if you find something wrong that needs to be corrected please chime in (but without attitude). I'm just trying to help. Also I know that these "fixes" don't always fix the problems but problems but they are just suggestions to help a noob have a great first experience.
ULTIMATE DROID 1.0.0
P3DROID kernel (forgot wich one though. lol)
Reds Urban Version 6.0 theme