Why do people use the word 'Best' i seriously don't get it.
There isn't anything such as best. There isn't any Monopoly by any of the software's in this category.
For some Vuze (formerly Azureus / Azureus Vuze) might be the best. For someone else uTorrent will be the best.
Reason's being different. One may have less HDD so he uses uTorrent which doesn't take much memory either. Other person may have extra storage and high end memory so he uses Vuze which gives you lots of HD content as well. So there isn't anything such as the best. It's what is best for 'YOU'.
Anyway ,
You have many options in P2P but some of the most popular one's include-
Limewire , Ares , Bearshare etc.