BITLORD: great client, great options, multiple trackers possible, cool statistics, connection and NAT problems
Utorrent: very small and using hardly any PC power, connection and NAT problems
ABC: not so many options, multiple trackers possible, connection and NAT problems
and now I am using AZUREUS for a long time, since I have the pc power to not mind this java monster, it is great, has great connection, but does not support multiple trackers (at least I have not figured out how)