I love this kind of discussion and exchange of points of views! Thanks for starting the thread.
Before starting my contribution I want to mention that I experimented a lot with various films, until I got fed up with it (and needed space in the fridge), so I decided to concentrate on the following films only, for consistency and predictability (knowing how the film reacts to various exposure conditions etc.) as well as avoiding filling the fridge with films I will probably never use.
Here's my medium format list:
Velvia 50. The main reasons are 1) I love the look and the colors, 2) I love projecting slides. It is my go to film for landscapes & travel.
I imagine the new E100 could become my new general purpose slide film at some point (it's probably more forgiving with people than Velvia), but I forbid myself to try it until my 20 remaining Velvia rolls have been used.
Portra 400. Terrific for portraits, skin tones, both in flash and outdoor lighting. I haven't been traveling much recently and got more into portraits of my family, therefore this has replaced Velvia in my most used color film ranking.
Tri-X, for the looks, the classic grain, the tonal response. I could probably use something else as well (HP5, etc.) but when I decided to stick to 1 type, Tri-X won.
I tried Fuji Acros and loved it, but prefer 400 ISO for handholding medium format. Grain isn't an issue in MF, and in 35mm that grain is lovely!
Provia 400X, for the (silly or not, you decide) reason that I stockpiled a bunch when it was discontinued. I make a roll every now and then, not sure I like it that much but the 400 ISO is surely a blessing with medium format! Next on my to-do list: try some portraits and compare with Portra.
My 35mm list is much simpler:
Mainly Tri-X, some Velvia, just tried a roll of E100 (I do not have any Velvia 35mm inventory!), will have the pics next week. I'm really curious about that film!