I don't really have a "favorite", but I do use a bunch. In addition to the ones already linked, you can try:
http://jorgeoscuro.googlepages.com/
OOO, it's a major overhaul and it's best to start games specifically for OOO. Personally I use OOO-Light, since it lets you customize which parts of OOO you use and which parts you'd rather keep vanilla (I drop the slow skill leveling but keep all the magic, enchantment, skill perks, and increased guild difficulty, for example). Plus you can play a hybrid vanilla game with OOO tweaks if you really want to if you get Light (just don't enable the OOO master file when you start playing).
http://obse.silverlock.org/
OBSE. Technically not a mod, but it's required for lots of great mods, and a big help for lots of other ones too. The Power is Yours!
http://wrye.ufrealms.net/Wrye%20Bash.html
Wrye Bash. Also not a mod, but a great tool with a number of uses. Mainly there to manage mods and conduct the occasional savegame repairs.
http://www.tesnexus.com/downloads/file.php?id=2299
No Psychic Guards, simple and sweet. Stops guards detecting silent crimes from twenty miles away, and makes a life of crime far more realistic. Much more risky, too, since it (optionally) increases the penalties for getting caught. If you want to do the DB and/or Thieves Guild, don't leave home without it!
http://www.tesnexus.com/downloads/file.php?id=10168
Exnem's Female EyeCandy, nude or non-nude depending on your preference. Changes all female meshes and textures to a more, ummm, "endowed" setting. There's tons of armor and clothing mods that rely on it, so if you plan on downloading lots of new equipment mods, this is a good choice.
http://www.tesnexus.com/downloads/file.php?id=10643
Almost required if you get Exnem, mods (almost) all vanilla and SI clothes to work with Exnem's new figure. If you decide to download Exnem, get this right away.
http://www.tesnexus.com/downloads/file.php?id=6239
Actors in Charge, not really necessary, but fun and lets you get some AWESOME looking effects. Gives you control over NPC and PC animations, so you can make Adoring Fan walk like a lich or get your character to pray at altars.
http://www.tesnexus.com/downloads/file.php?id=4431
Rens Beauty Pack, very popular for some reason, but I hardly ever use it. Adds in a crappy little elven race and some interesting new hair, as well as new eyes for the elves. Also good for the same reason Exnem's is, i.e. lots of addons. BE WARNED: don't try downloading any of the "bundle packs" which claim to merge Rens with tons of other good mods. I tried one once, and wound up with Cyrodiil full of Orcs who looked like Botox zombies and effeminate Nords who resembled those overpriced Japanese dolls. Just stick with Rens.
http://www.tesnexus.com/downloads/file.php?id=3644
BodyLifting Package. Do you like to drag bodies? Sick [&@%!]. So do I. This helps you do it.
http://www.tesnexus.com/downloads/file.php?id=6021
Decorator Assistant. Do you like to decorate houses? Then you need this.
http://planetelderscrolls.gamespy.com/V ... ail&id=416
Fatality Cam. Do you die a lot? Then you... well, you don't need this, because it only works when you're dead. But it's fun. Jump off Dive Rock and watch the show!
Also, there's another "mod" which isn't really a mod, but I can't for the life of me figure out how I did it. You can remove the world boundaries with some simple tweaks to the game files. I did it myself a looooong time ago, but have since completely forgotten how I did it. Searching on Google turns up nothing but page after page of the same bloody review sites mirrored 50,000 times, so if anyone knows what I'm talking about and can post a link to the method, it'd help us all. It's tons of fun to run around Skyrim and Valenwood, and it's prime real estate incase you want to make your own mods, since, well, it's not got much in it and nobody puts mods way out there.
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