I prefer meat. A Ligurian restaurant near us serves the most marvelous ravioli in brown butter, filled with oxtail meat. Yum!
The other filling I prefer is polenta. I always thought that was the oddest combination, but a marvelous Northern Italian place opened up in town several years ago. The owner/chef is from Bergamo (near Milan) and makes a wonderful pasta dough with extra egg yolks, so it's extra-supple and flavorful. I tried his Tortelli Di Polenta Timo E Coniglio: Egg pasta stuffed with polenta taragna, rabbit sugo, thyme, Grana Padano D.O.P. - and was in love! Even my meat-loving spouse thought it was sensational.
Polenta taragna is a mix of polenta, butter, and cheese. Needless to say, the quality of polenta is critical - true flint corn, preferably stone-ground. Anson Mills is one of the best domestic polentas we've found, but there are a few others still in business.