well the title says it all.
I have been using blender since 2.57(2.49 a while before that) and i’d have to say my favourite features would have to be how much better appending and linking is, and of course, cycles.
how about you?
(sorry if there have already been a lot of thread like this 
)
Since I started using Blender? Undo
Either the UI overhaul, Bmesh, or Cycles.
I started somewhere in the mid 2.4x releases so it’s difficult to choose because the entire program has basically been completely overhauled since then. It’s pretty fun to go back and think about all the stuff that has happened though. Even with all the ups and downs, it’s really been fun being a part of the community and watching it grow. The next 4 or 5 years should prove to only be even more interesting.
Definately Cycles
undo? LOL go Fweeb! 
Definitely the UI upgrade from 2.49 to what it is now.
I can still picture 2.49 in my head.
Ehhh…
IMO the new icons since 2.5 are totally the coolest new feature. By far.