My last creations, 2 large (very large!) trees I've made for TF2 (ep2 engine), I'm very sattisfied with them because they look very TF2-ish and the shadows it makes on the leaves are "realistic". I'm also very sattisfied because it really feels like a dense leaves, on the second one you can't see the sky from below. And they're not too high poly, 6k each one (aprox). But I'm going to do lots of lod models so it will go from 3k to 6k.
Pics of the first one, which I like the most and you can climb on it because the trunk is sliced: 2nd tree: Oh yes I forgot to say textures are ripped from Crysis and modified (to a TF2 style). Don't blame me for doing that.