What are the best ones available?

Personally, I don't use ones that add sounds to cities etc. so I can't give any advice on those. I find them immersion breaking, as in "where does that sound come from?".

I always use http://www.tesnexus.com/downloads/file.php?id=13987, though. Excellent overhaul of the combat sounds.

Probably you can use one thread to ask all these best of questions.

Also search the word azerbagb (I think it is) + whatever topic you want to find with regard to best of. Then of course there is this http://www.gamesas.com/index.php?/topic/1010109-list-list-of-mod-lists/.

Sound mods I use (and altogether):
http://tesnexus.com/downloads/file.php?id=5861
http://tesnexus.com/downloads/file.php?id=9218
Symphony of Violence. Link above
Atmospheric Oblivion (get http://www.tesnexus.com/downloads/file.php?id=19487 and install after http://tesnexus.com/downloads/file.php?id=7703).
http://www.tesnexus.com/downloads/file.php?id=19038
http://www.tesnexus.com/downloads/file.php?id=12993
Cliffworms Oblivifall - http://tesnexus.com/downloads/file.php?id=29676 a town version coming soon!

For NPC voices I recommend http://tesnexus.com/downloads/file.php?id=9829
Best rain and thunder sounds - http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=4400 I install this over weather overhauls even.

Also worth reading is the sound section of the http://devnull.sweetdanger.net/other.html. Some are outdated though so search for the name of the mod on Nexus for more recent versions.

And and outdated http://www.gamesas.com/index.php?/topic/957424-custom-bain-projects/page__view__findpost__p__13861937 I describe in my old thread. It would be updated with the inclusion of some of the above.

There are a few OBSE plugins that utilize advanced sound control, but I've had some frustrations with them to be honest.
http://www.tesnexus.com/downloads/file.php?id=16288
http://www.tesnexus.com/downloads/file.php?id=17050
http://www.tesnexus.com/downloads/file.php?id=19772

... I defy a more complete answer.

my vote goes for combat sounds 300