I don't believe there is one best sword. On paper, the german greatsword seems awesome and does indeed work well. However, there are two issues with how I play that make using that sword less effective. First, I am a fairly slow strength build(mistake!) so if I miss, I am fucked. Second, I always tend to hug my opponents and with the longer swords, you need to take a step back. I have a hard time doing that.
So, I tried different weapons since I got nothing else to spend my cash on and I find a morningstar to be working pretty well for me. Its faster and longer than a great maul, which is so damn short and slow. But its short enough that I can still hug my opponent and be effective with it. Since I have stength and iron flesh, a morningstar works best for my style. Get in their face and smash their shields. You're too slow for much else. I just smash and smash til I get overwhelmed or arrowed to death.
But it all depends on your style of play. If you can step back and dance and are good with manual blocking, get the german sword or whatever is longer and feels right to you.