I prefer Trill. Not completly immortal, but being able to change your life and experience new points of view and accumulate several lifetimes of skills, relationships and knowledge. I think their lifes would be more interesting and potentially they could be the wisest of all the races (seriously, would anyone want to be an El-Aurian)?
Q has his obvious appeal, but the continum seem very 'controlled' and finitie to a degree. I think that would drive you mad after a while. No point in working hard, as that next step will be always out of reach.
Vulcans with thier enhanced intellect and strength, but limited emotional intelligence? No thanks.
Betazoids - too touch feely for me.
Klingon? Would be fun for a bit, but no.
Changelings perhaps? I'm sticking with trills.
I'd probably be Romulan. I have an appreciation for a highly structured/ordered society. Since they're descended from Vulcans, they're highly intelligent, and might possess the advanced strength(although, oddly, that was never touched on in the series).