Depends on the task.
Just GM or the one that does program changes?

The top ones that I know of that do program changes are:

The budget, and the one that you probably already have is VSTsynthfont, which is very flexible. Quality depends on SF2 bank you have.

The best, in my view is Halion / Sonic from Yamaha. Comes as VSTi or stand alone software. Prior to this year, it had VST2 version, which works great in BIAB. As of now, only VST3 (and standalone). Peter Gannon confirmed in wishlist that VST3 support is coming next year, so I will hold off on upgrading for now.