Limewire and Frostwire use the gnutella network, which, I feel safe in saying, isn't the best of all networks available.
It's handy and simple to use (excellent if you're coming from Kazaa and want something just as simple to use), but not excellent.
Bitcomet and Bitlord both use the bittorrent network, for which the best client is, hands down, Azureus.
There are other networks, mainly eMule/eDonkey and dc++/RevConnect.
The old FastTrack network (the one used by kazaa and its various offspring) is still online, but you'd do well to avoid that one.