Depends on your play style. Do you want slower but hard hitting greatswords or faster stabby swords? Some of the boss souls can be used and turned into some cool swords. The drakewing ultra greatswords is pretty nice which you can get after beating the guardian dragon. The standard greatsword is also nice since you can put infusions on them and enchant them as well. Zweihander is also a fairly popular greatsword. It's got a bit of versatility for a bigger sword and hits pretty hard. You can get this one in iron keep. If you go from the smelter demon's door and go straight down the path ahead til you see some broken stairs on the right hanging over the lava. If you jump on there, go in the building and climb the ladder, zweihander is up top. For the faster stabby ones I really prefer the sun sword. It has nice stats and levels up nicely with strength and dex so it's a good sword all around. For this sword you need to join the sunlight covenant and pass in 20 sunlight medals. You get those by either joining someone's game and helping them fight a boss or having a sunlight covenant person join your game and kill a boss for you. It can take a bit, but it's a great sword with a lot of versatility. My personal, all time favorite sword is my lovely moonlight greatsword. To get that though, you either have to go to new game plus or use an ascetic to make a certain boss harder. You also upgrade it through your intelligence level.