.45 Colt, due to cost and availability. I wanted something with more oomph than cowboy loads, but I'm not using a Ruger either. It's what got me from just a passing interest in reloading to actually learning to do it.
Second is .38 special. I own more .38s than any other caliber. One of them is older and while factory jacketed probably won't hurt it, I feel a little better with lead over a conservative load of Unique. Again, it it necessary? Probably not, but I have the means to produce it so why not?
Both of these have gotten me interested in casting as well, but that's down the road a piece.
To be fair, I am still VERY new and somewhat green to reloading but am head and shoulders above where I was a year ago. A year that has included very little shooting, but I'm getting there.
With an eye to the future, I have eight pounds of IMR4895 and a set of dies for .30-06. I'm not really ready for rifle loading yet, from a comfort or equipment standpoint. By the time I run out of M2 ball, I hopefully will be. Loading for my Garand is what is going to get me there.