Depends on the situation. Small trees without a serious back lean I'll usually use a retired half inch climbing line. For larger trees I will go to either my 5/8 or 3/4 bull line. If I am dropping a 36" 100'+ diameter tree I am going to use the longest, heaviest line in my arsenal. I like using my 200' 5/8 bull line on trees like that... On large diameter spars I like to use my shorter 3/4" bull line. Especially if I am going to put a MA on it... You do know what a MA is right???
The heavier and longer your line is the more leverage you are going to be able to get...
If I am throwing a pulley or two into the equation to pull something heavy over with a lean I will use my 3/4" bull line. Maybe something like this: