Ask me this before Friday that passed, and it'd be a tossup between the Tamron 70-300mm and my DA10-17mm Fisheye Zoom

Now, it's a lens that i don't have, but got to spend a few hours with. the Pentax DA* 50-135mm.

Fast focusing, instant manual focus control, nice colour reproduction, and Weathersealing to match my K200D body.

Borrowed it off a friend from another forum for a meet in the city. Was just a wonderful lens to use, made even more so given i was shooting in the rain