This thread pops up literally every day, ALWAYS with the same responses. There's a sticky for this, which I'm sure contains ALL of the same answers you're getting here.
That said, I'll give you the generic opinion everyone else does:
Rapier = best all around
Longsword = bossing, but I use rapier more even for bosses because it's higher DPS
Maul = Good for pking, but gets boring after using it for a while + people safe their asses off, you kill yourself with veng a lot, etc.
I own all 3 and if you intend on only getting one, the rapier is the best choice all around because it can be used for PvM AND PvP and still be VERY effective. However, I really would just suggest getting all of them, so you never have to pick.. but I know there's some people out there who just hate DG.