Teamspeak is,IMHO, by far the best. It is the least intrusive followed closely by Ventrillo. Skype is VERY intrusive and will cause lag. Roger wilco is ok, but the least in terms of sound quality.
Another benefit of TS is that they have a beta server you can use to set up your room so you don't need someone to set up a server to run it. We have a default room that is password protected. When someone, anyone, logs on, they check to see if the room is there and if not, creates it.