Tamron 70-300.
But then it is the only lens I have as the kit lens for my D40 bought from Bennets last March imploded within 5 weeks and they refuse to replace it so until I sue them I only have my Tamron, it was the only lens I could afford so that I could use the camera at all.
I do like the lens though, it`s been great so far for learning with.
I would like a longer lens for wildlife/birds but the Tamron does nicely for my preferred targets of dog agility and horses, plus the Macro [ well, close up ] option on the lens means I can get reasonable-ish shots of flowers/berries etc as well, so as a duel purpose lens it`s not bad at all.
[ I am a beginner though so I don`t have anything to compare it to really other than the crappy duff 18-55 which I only got to use for such a short time that I did`nt get to explore it`s potential ].