Is torrent a p2p? LimeWire is popular but what with everyone getting sued these days I think most are avoiding them
As far as the whole torrent thing goes, then definitely uTorrent, as Felu says, but if you're looking for a LimeWire substitute, I would go for SoulSeek. The best I've encountered so far, after using quite a few; not to mention, easy to use. Whatever you use... don't ever, ever, ever... download Riyocodec, no matter what they tell you in the "Must Read".
Bittorrent using uTorrent and protected with PeerGuardian 2 is the best p2p setup without using proxy servers. There is way more stuff being shared on bittorrent than limewire or kazaa, especially if you're looking for bigger files. Also, because of the way bittorrent works it can be a little trickier for the RIAA or MPAA to sue you if you happen to be sharing one of their songs/movies.