Which greatsword is more versatile?

no its a curved great sword o-o, its r2 is a really heavy thrust

That's one of the problems with the bastard sword in PvP, is it's incredibly vulnerable to the claymore. Which is one of the most common weapons. They have the same attack speeds, similar stamina usage, the moveset is just a little different and the bastard sword trades extra damage at the cost of range. When you're in close and personal the damage will eventually win out, but that claymore poke is just...yeah you already know lol.

There are a lot of options, though. I've personally barely touched the flamberge, mostly on a account to my love of katanas IRL so those tend to be my preferred bleed weapon, and the fact that games have a tendency to make flamberges greatswords, when they were historically mostly rapiers.

I've toyed around with the black knight sword but it just feels awful to me. I can't find any justification to use the R2 attacks, which limits you horribly. Being that my faith and int are very low, I haven't bothered much with the Artorias swords either, though they seem to theoretically be the best weapons in the game at high Slvl. If you're doing some level 200+ PvP those might be worth checking out.

Has anyone played around much with the gwyn's greatsword? I haven't actually made one yet so I can't comment on that. I never seem to see anyone say anything about it.

Also another thing to consider, the Murakumo is a curved greatsword, that one feels pretty versatile to me. Maybe give it a try? One might also consider the washing pole an honorary curved greatsword too.

It was asked what Greatsword is more versatile.
Definitely Claymore.

C/C scaling makes it usable by both Str and Dex characters. It has typical Greatsword moveset, but it can stab with 1H R2, which is usefull in narrow corridors and does piercing damage (some enemies are resistant to slash damage ya know).

Flamberge is also versatile, as it combines moveset of Greatsword and Curved Greatsword + it has bleed, which is a big plus. However it's meant more for Dex builds thanks to it's scaling, so others have to use it elemental, which isn't always ideal.

MLGS is awesome thanks to doing pure Magic damage (but some enemies are rsistant to it) and has very usefull R2 attacks. However it's only usable by Int characters.

I'm not saying which is better for that, or that, but when it comes to versatility and versatility alone, Claymore wins.
In my opinion, at least.