The second. It's the simplest and cleanest. Maybe on a nice lime background with a touch of juice drops on top :P
But seriously, the second is my favorite.
You should really have added a poll to this page.
That being Said, My first choice would be #1 (Looks realistic and downright cooler than the others)and Second would be #2 (For Clarity and simplicity). Also try adding a Green Shadow to the logo. Somehow all yellow seems... well... not good
I would go with the first logo, its simple yet the lemon looks very attractive.
The second one looks very simple.
Last choice is the last one.
The "U" is not really emphasized