I can't really pick one for various reasons.
XP2+ if I could get it in 4x5 sheets, but I can't.
Pre-2007 Tri-X if I could get it fresh, but I can't.
At best I settle on a few: FP4+ for medium speed, HP5+ for 400, because I can get both in all film sizes I shoot, 35mm, 120, and 4x5. So that would reduce it to two films and if I had to pick one of those, HP5+ because I don't shoot much 35mm black and white at all and in medium and even more so large format the grain difference is far less important, if even noticeable. I also very, very rarely shoot in bright sun. As in almost never, maybe a vacation twice a year. Since I work overnights and sleep through the heat, and bright, of the day most of the time, by far most of my shooting is from later afternoon into twilight, or sometimes in deeply shaded woods. Either way, I prefer at least 400, though for the times 125 will do FP4+ gives me superb results. So if I really HAD to pick just one currently available black and white film it would be HP5+.
Fortunately we don't have to make such a choice.