Unfortunately (or fortunately, depending on your perspective) there is no such thing as "the best UI". While people can point you to the mod they prefer, you have to remember that it is just that, a personal preference. Person A prefers Mod/Compilation X. Person B doesn't like Mod/Compilation X, they prefer Mod/Compilation Y. Person C uses stand-alones only, and mixes and matches to get their desired result.
CTMod, GyspyMod, BloodEagles Recompilation, FlexBar, Insomniax Recompilation, Cosmos, SvartenUI, UltimateUI (UUI), BibMod, Telo’s Mods, MonkeyMods, MiniGroup, Woofiest Compilation, etc, etc, etc.
(Please note, this is definitely not a "complete" list, there are many many others, and the ones I listed aren't in any particular order.)
You have to figure out which one gives you the options you want, or mix and match to get what you want. Check out different mods, find what works for you.
Mod Hosting sites besides WoWInterface.com:
WoWGuru:
http://www.wowguru.com/
WorldofWar UI section:
http://ui.worldofwar.net/
Curse Gaming:
http://www.curse-gaming.com/mod.php
Cosmos:
http://www.cosmosui.org/
WoWAddons:
http://www.wowaddons.com (defunct?)
WoW Database UI:
http://wow-db.com/filelist.asp (defunct?)
And, of course, the sticky thread at the top of this forum:
http://forums.worldofwarcraft.com/th...tmp=1#post2739
Plus all of the mods that are self-hosted, most of which can be found by browsing through this forum.
There are also lots of macros out there that can help make your life easier as well.
You can find a lot in another sticky thread at the top of this forum:
http://forums.worldofwarcraft.com/th...tmp=1#post2739
As well as at these sites:
http://www.wowmacros.com/
http://www.wowwiki.com/Macros