I can understand why Mhen'ga is popular. It's the first planet, so everyone knows it well. Additionally, it's got a mostly natural environment (for that "unexplored land" feeling) and a decent variety of opponents.
Tarkus isn't bad, but it's full of junk and rust so I wouldn't want to imagine what it would be like to be there.
I don't like New Texas. A planet full of cowgirls in a mostly natural environment (prairies, some forests) is actually pretty awesome, but the whole Texan theme is just too silly and ridiculously out of place and it just ruins it for me.
I like the caverns of Myrellion as an environment, but... bug-people everywhere? It's simply not my thing.
I tend to like cold settings, so Uveto might be of particular interest to me. Haven't read much about it, though, so I don't really have an opinion at this time. I'd be excited about it, but I know it's probably going to take quite a while before it's added.
I'll refrain from voting for now.