Dante or Sonsie
Dante's hands down for the quality of food, attention to detail and you can have good views of the Charles River even from the inside.
Sonsie is definitely a people watching, trendy-ish, beautiful people type of joint. Not a big favorite of food aficionado's but it does have it's followers.
Burton's is an "expense account" type steakhouse place. Not as expensive as other steakhouses in/around Boston but also not really a memorable destination either.