Chris Lopez is a great trainer with an informative blog. He has a good post on The 6 Essential Exercises of Kettlebell Training and How To Program Them Into Your Routine. His "Top 6" are...
-The Swing (1 arm & 2 arm)
-The Military Press
All in all, a great list. Personally, I think the Long Cycle Clean & Jerk may be the ultimate kettlebell move, but that's a minor point. Chris's post is well-worth checking out.