Homecoming is actually an official server, licensed by NCsoft. The others are unofficial. As for best, that's entirely subjective. Homecoming and Rebirth have added new content. I'm pretty sure that all of them have added new power sets. Some have added entire new Archetypes.

If all you care about is population, then Homecoming. If you want something really close to the retail servers but with some fun new additions, then Rebirth. The rest are pretty tiny population wise, but have some wild additions and changes to the retail game. I'd suggest giving them all a try to find which best scratches your City of Heroes itch.

There are several servers, with Homecoming being the largest. It generally has a peak concurrent population of about 2,000 people these days.