I've always completely overlooked the dex greatswords. Are any of them good in any of the games? I'm playing DS2 right now with a str build, using a greataxe but planning on switching to a zweihander or something. Any recommended weapons?
Well, for starters, Zweihander is a quality / strength focused ultra greatsword, not dex. It may start off with a C in dex and D in str, but it will never gain an increase to dex scaling, while str will jump up to B once it hits +10.
Its a similar case with the Ivory King sword, where it starts off with a D in str and B in dex, but when you fully upgrade it it becomes B/B, making it more of a quality weapon.
If you want a dex ultra greatsword, you'll want the drakewing, which is supposed to be one of the best dex weapons out there.
If you mean just greatswords, there's actually a surprising number of them that are dex based rather than str based.
Drangleic, Llyoce, Charred Lyoce, Mirrah, Flamberge, and drakeblood all favor dex builds.
Of these, the Llyod and Mirrah greatswords are pretty nice. Royal and Flamberge have a bleed gimmick, Drangleic is heavy and needs twinkling, and drakeblood is a cool sword, but it wants petrified and has built in split damage.
Charred Llyoce is also pretty cool and its easier to get than the Lloyce, but it has lower dex scaling and built in fire damage.
Of course, Curved Greatwords typically have high dex scaling, with the exception of the Nil sword, which is strength based.