Amis or Osteria
Definitely Osteria. Amis is great but less memorable.
At Osteria, you can't go wrong with the best pizza in town and a couple half portion pastas. You might want to call and see if they have the pit roasted suckling pig special - my favorite.
If you're not on a budget, VETRI hands down.
I've only been to Amis, not Osteria. I like some of their dishes very much - mostly the pastas, meat dishes and desserts. Almost everything else I've found I've gotten much better preparations elsewhere in the city at this point - better prep, better taste, better portions. Was very disappointed in their Roman Artichokes as much as I wanted to love them, based on how much I did in Rome. The seafood mixed grill was kind of a joke and way too oily, compared to the excellent ones I've had at Melograno and Salento. But the ravioli I had there on last visit were amazing and the desserts are out of this world good.