Live trace is the best! also pathfinder :)
pen tool and mesh tool
distribute, envelope distort, recolor.
pen tool and mesh tool also
The Appearance pallet and Transform Effect get quite a bit of usage from me.
Definitely the pen tool, if that doesn't count blend is very handy.
From all of the features I may say that my favorite is everything concerning transparency. You can do a lot using just transparency effects and masks.
the plugins feature is by far the best.
it alows you extra features like the new astute plugins that makes illustrator the best vector program that is.
The Live trace for sure. Lets you vectorize ANYTHING
The Pathfinder is probably the most powerful tool in Illustrator. With a little planning, you can make complex shapes out of basic shapes. It can save a huge amount of time.
Pen Tool, Live Trace, and Pathfinder, depending on what you need to do.