How much solo power do you want?

PLD currently has the most solo power and a rather active rotation that always feels like you're doing something.

DRK is the most active with the most oGCDs still but less useful for solo.

GNB is second most active, but has a pretty good solo power, better than DRK's.

WAR is the least active but very powerful overall. Good solo prowess, not as good as Paladin on raw survivability, but doesn't lose damage for its survivability and is more survivable than GNB and DRK.

In the current time, which is what you're asking for, right, I'd go with PLD or GNB if you're serious about some old content solos. But if you just mean like, solo some fates or whatever, DRK.