I have been very indecisive about avatars before. For my first year I used a variety of Rorschach avatars eventually culminating in this beauty [http://s12.postimg.org/cq5zpuzo9/Rorschach_Tophat.png] courtesy of the combined work of [user]Fanboy[/user] and [user]nope.avi[/user].
After that I rotated through a few avatars, and eventually became fond of putting Awesome Face on things after being inspired by an Escapist News Network [http://www.escapistmagazine.com/videos/view/escapist-news-network/1775-Resident-Evil-Romance] episode. After a few conversations this resulted in my unintentionally making The Awesome Spy [https://pbs.twimg.com/profile_background_images/241099757/Awesome_Spy_Background.png] (or 'teh awesomespah' depending on what mood I'm in) in mid-July 2010. That's easily been my longest-running avatar.
I've had a few different avatars since then, but most of them have revolved around either sticking Awesome Face on things or sticking things on the Spy, depending on what is easier/more suitable. I'm not sure about how long for one avatar specifically, but I've kept to this general trend for roughly 3-4 years, ignoring some other short lived avatars.
As for favourite avatar by someone else? Too many to choose from, though one that comes to mind right now is [user]Formica Archonis[/user].