That depends on what you're planning on doing. Melee DPS or Straight Tanking?
Let's put it in COH terms -
If you're wanting a more "Scrapper/Brute" style of build, you'll want Unstoppable. It provides less overall protection than Invulnerable, but raises your melee damage (all types). You'll still be pretty durable. But large DPS Spike damage against you will hurt. If you're running freeform, then you want to be in "Brawler" stance.
In my experience Invulnerability is your straight Tanker ability. This is what you want to have on to either take all the damage you draw to yourself via aggro (assuming you have the capability to draw the aggro, but that's a different topic) and just take it and survive. Allows you to pretty much ignore "trash" mob damage or anything coming from enemies with levels significantly below yours. Synergizes superbly with the Tanker stance.
Me? I like having both options available on my melee toons. I'll put Unstoppable on the Brawler build, and Invulnerable on the Tanker build tabs. Generally run in Brawler by default. But Switching to tanker for "survival" mode in harder fights.
Regeneration I haven't used much, just run some basic experiments with. So take what I say with a grain of salt. Seems to operate similarly to the way COH regeneration did, with all that implies (both good and bad.) I'd suggest using it on toons where you're going to be going for stacked self-heals. Make it the base of a true regeneration toon. Probably as a theme like a lycanthrope or a Wolverine type.
