Luc,
I like Kodak Tri-X 400.
With my Pentax 6x7 I had always used just the slow stuff (for both B&W and color, actually). But recently after getting interested in more "basic" MF cameras - including toy cameras (I have a Holga) and vintage cameras with simple meniscus lenses - the fact that I use these almost always hand-held, and the fact that I don't plan on printing the resulting images large have encouraged me to try faster film.
What attracted me to Tri-X for those uses was that it was considered a kind of classic film with a traditional "look," and that it was easy to find (and hopefully will be for a long time). What's more, I liked the idea that I could develop it in Diafine for a speed boost, and also increase the exposure range over which I could get usable results. That last point can be very important when using cameras with such limited exposure control.
My main qualm about it was that I have usually had to rate it at 200 ASA when developing in my standard developer, HC-110. I just couldn't seem to get good shadows out of it when rated at 400. Hmm, maybe that's what that traditional look is all about!
Also, it seemed a bit grainier than I would like if I were to ever try it for "serious" landscape work in which I would want to print large.
However, just recently I had some very encouraging results with Tri-X and semi-stand development in HC-110 that seem to overcome those qualms. The grain was much finer and smoother than I have seen previously, and (pending some further testing) it looks like I am getting at least rated speed out of it, and possibly up to one stop more. I hope I don't eat my words in saying that, since I really need to scan a test roll I took and verify that I can duplicate the results before I am really sure. But if you are interested I can provide details. I would also be very interested in hearing about your development techniques with the Efke 25.
Jeff