Favorite favorite? Not sure actually, but here are some that I use in every single playthrough.
Painkiller's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6383 -best shield replacer ever made imo.
Painkiller's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6655 pack -best replacer for drakes imo and it comes in a lot of versions to suit many different tastes.
And finally Painkiller's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6768 - awesome compilation of various stuff including short but not any less witty dialogues, tapestries, talking dragon made by Phijama, and of course, great variety of swords and shields, inspired by LOTR, Deathdealer and others. When I come to think of it, if any mod is my favorite this is probably it.
Also Darknut's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=5882 - texture replacer for all the armors. I think there was a mod somewhere that adds bump and reflect maps and all that jazz but I never got to DLing it for some reason so I still use this.
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6143 - same as above but for all the weapons (that is a couple of weapons that I didn't replace hehe).
http://download.fliggerty.com/download-113-317 - even though I don't use it in every playthrough, I do use it in most. I love kids (yeah I'm young, don't ask
).
http://lovkullen.net/Emma/lokken.htm - really awesome quest mod, probably my favorite of the type.
Abot's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=2786 - adds birds!
Also Abot's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=258 - adds fishes.
Arcimaestro Antares' http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=136072&id=6777 - adds all kinds of undead, including jumpy skeletons, screaming banshees and more.
Almost all Vurt's stuff I ain't gonna link since there's too much and everyone know these anyway (at least I hope so).
Vality7's http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=5897 and http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6316 - I still prefer these over Vurt's tbh, and Balmora has this unforgettable feeling with it to me.
Also his http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=6306 - adds nice bumo maps to ice cave set, which I was opsesed with back when I didn't know how to do this myself. Still love it!
And of course Better bodies, Heads and Clothes which I also never play without. There's probably much more but these are some from the top of my head.
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=9388. Finally adds an inn with a rentable bed to Seyda Neen.
Hah, I'd never expect to see such a small addon in anyone's favorite list, thanks.