bagnon/banknon is awesome for changing the look of bank and bag windows and extremely useful as well as it saves space on the screen.
Skinner is great for changing around the look of your windows like AH and character screen, to make it more futuristic
Bongos is an EXTREMELY useful replacement for your usual action bar. it can save you lots of space on the screen if you have only a small resolution or make your screen look even bigger if you have a big one! it make it easy to lose all the bars that leave you only with a little hole to look through
another staple imo, is Pear Classic or X-Perl unit frames. its hard to decide which one is better as this relies a lot on personal preference but these 2 are definitely the best unit frames around.
FuBar, the worthy successor of Titan Panel adds a huge variety of functionality to the bottom or top of your screen in just a thin bar! it gives you countless different types of information and the plugins are always becoming more.
another great addon would be CT mod. probably one of the most famous, its a whole library of different functions and quite versatile
finally, i think its pointless to try someone elses interface or download a pack and use it as every player sees different things as important. You should visit sites like www.curse.com and just take maybe an hour or two of time to find the best addons for yourself. Theres much much more than what i have listed here and i have much more than that too, its impossible to list all the good stuff, youll just have to spend an evening getting your perfect interface together
. thats what i did and now wow is twice as fun