Mushrooms are the best, I think - there is something about mushrooms and eggs which really work well together. If I have a lemon, I will grate a little bit of lemon zest into the mushrooms - delicious.
And not forgetting cheese, of course.
Actually I more often make a frittata than an omelette. I don't have a conventional cooker at the moment, but I do have a Remoska, and it is very easy to make a frittata in that.