2400, hands down. If I could have a second powder, either Bullseye, Red Dot or 700x would suffice.
Why 2400? Because I can load everything reasonably well with it. .357 and .44 Mag, .45 Colt, and it's the only powder I use in .45/70. I've loaded 12 guage with it, and it works well. 16 and 20, even better. For .410 it's perfect. Works well in .38 Special and .32 SWL; also .32/20. Very, very good in .30-30, reasonable in .308 and even .30-06. Very good in .223 too, and I can even tune an AR-15 to function with it.