I also use the RB67 ProS, my favourite camera.
I also like Tri-X, just not in D-76, I really like Xtol.
I can't recommend what'd be 'best' without what you intend to use.
I often find higher speed film is good in the RB67, due to the slower wide-open aperture on the lenses (generally f/3.5 or f/4.5) on the RB67.
T-grain films are your standard call for fine-grain and sharp... though on 6x7cm it's less important as you'll still have plenty of resolution to work with, and Tri-X can be fine grained enough depending on development, very nice in Xtol, 1+100 stand/semi stand in Rodinal keeps grain fine, while you can make it quite coarse in 1+50 if you like that.