The Founders, for sure, they hated everything they didn't control.
The Tholians could be argued to be as, or more, xenophobic, but I think it depends which way the wind is blowing at the time. Looking at a variety of sources, they're either extremely xenophobic or happy to associate with the Federation (The Tholian Ambassador gave Sisko some Tholian silk for Kassidy if I remember rightly).
(Off topic: More Tholians please! Not enough about them in any of the series!)
The Tholians could be argued to be as, or more, xenophobic, but I think it depends which way the wind is blowing at the time. Looking at a variety of sources, they're either extremely xenophobic or happy to associate with the Federation (The Tholian Ambassador gave Sisko some Tholian silk for Kassidy if I remember rightly).
(Off topic: More Tholians please! Not enough about them in any of the series!)