Where to start...
Some say his Mass in B minor is his finest work.
http://www.youtube.com/watch?v=BraHTI8jZtY
The cello suites, of course.
http://www.youtube.com/watch?v=KHzfD6XLK7Q
I lean more towards the well tempered clavier (WTC) BWV 846-893. You should listen to the entire WTC , they are all really awesome. Glenn Gould has a really excellent and unique rendition. Andras Schiff and Angela Hewitt also have great versions of it.
Here's a couple of my favorite ones.
http://www.youtube.com/watch?v=eJkaw_Vwz0Y
http://www.youtube.com/watch?v=_N1JFXScmYw
Of course the harpsichord and violin concerto's are amazing.
They are BWV 1052-1058 for the Harpsichord, and BWV 1041-1043.
http://www.youtube.com/watch?v=eIWaL8qqiqs
As has already been mentioned, the goldberg variations are something else!
The St Mathews passion is also an amazing work, and happens to be the work that really introduced "Bach the composer" to the world. Of course this would happen decades after his death, when composer Felix Mendellsohn played it in the winter of 1819. This also happened to be the first concert which set precedent in conductors playing other composers' works. Bach, while alive, was mainly known as an organist in his hay-day, and his son's achieved much more success as composers than did he, while alive.
His finest work, maybe, is the Art of fugue, which he left unfinished, because he passed away. Contrapunctus XIV is one of his most amazing compositions, and his very last one.
http://www.youtube.com/watch?v=JbM3VTIvOBk
The french overture, AWESOME!
http://www.youtube.com/watch?v=I-SGd7C9-0g
and the french suites, Might wanna check out the english suites too while you're at it.
http://www.youtube.com/watch?v=w6PFS5o-gvE
The passacaglia and fugue is cool as heck too!
http://www.youtube.com/watch?v=1atQFLYbzuk
So is the little fugue!
http://www.youtube.com/watch?v=pVadl4ocX0M
Boy, i could be here all night.......