My top five. YMMV, that's ok, this is my list:
1. 9mm. I hated this cartridge b/c of its poor performance with crappy ammo. But many of the best pistols you can buy are designed for this round, and there are hundreds of loadings for this round, from .380 power up to .357 SIG.
2. .380. Yeah, I know, its a wimpy round. But let's not forget who invented it (rhymes with "Browning"). Anyway, I love my SIG p232 and without this round that gun would not exist. There are a number of other nice defensive guns in this caliber, too. I just like it.
3. .45 ACP. 1911. SIG p220. Need I say more?
4. .40 S&W. Yeah, its recoil is snappy. But every new gun that's worth anything is chambered in this round. Why? B/c its one of the best manstoppers in history. Doesn't look great on paper, but it gets the job done as well as any cartridge there is.
5. 22lr. Shoot all day for $10. World-class accuracy. Great small-game gun.
Honorable mentions: .357, .38, and .44 special. The .357/.38 revovler is the most versatile chambering ever. A generation ago I would rank it #1. The .44 special; let's just say a shoot shooter that makes a big hole gets high marks from me any day.
-David