9mm. Unique
Try Power Pistol. I think that ground has been covered.
.38 special. Winchester 231
Can't beat W231 (or HP-38 - they are the same) and it really shines in 38 Special and 45 ACP. If you have no trouble finding it, there's not reason to change. Seriously.
.357 magnum. Winchester 296 or Blue Dot
Hard to beat W296 (or H-110 - they are the same) for full-throttle magnums; especially with heavy bullets. I'm not a fan of Blue Dot. I found it finicky. Lots of good propellants in the same burn rate, but I have to give the nod to AA#9.
.45 ACP Bullseye
A classic combination. Hard to beat. "Substitutes" would be W231 (not new); AA#2, or TiteGroup. Between the two, I like AA#2 quite a bit better. #2 meters second to none. Runs tame and predictable. And even when turned way down, velocities are very consistent. Mind you, my experience with it is with 38 Special; but I have no doubt it carries over to 45 ACP.
TiteGroup is really spunky, has a really low fill rate (high density powder), burns hotter than satan's home below. It's not good for lead because it tends to lay down lead and smokes up the range from the burning lube. But it does make good, consistent plated (or jackted) basic range shooters. So it's certainly not without its merit. A lot of people like TG and it's abundant.
What's hot and what's not ?
That's about the best I can answer. I've been loading since '84. I've used many of the classics and the new stuff. The new stuff tends to be easier to find. But other than that, I don't see much compelling reason to change.
For me, the classics of W231, Bullseye, W296, and 2400 are fantastic and hard to beat. I am however, quite fond of the relatively new Power Pistol and the entire Accurate Arms lineup (#2, #5, #7, #9).