Grave Titan is head and shoulders above the rest. Sun Titan is sweet but some might find he provides too much value. The other three are fine. I only run Inferno Titan, and that's a blast to play with.
I feel like Inferno does too much honestly. It's a 6/6 dude for 6 plus you get a free DIVIDABLE bolt every turn, including the turn you play him. That's pretty busted honestly.
He's never been in my cube formally, but I've tested him a few times and he took over games in ways I was unhappy with for 6 mana. But admittedly this was small scale testing.