I used vent for about 6 years and I still very much like it. Good sound quality (though it varies a bit per host), easy to use if you take 5 minutes to read a basic tutorial and so, so many options.
In Ducks we currently use Teamspeak 3, which offers the same basic functions as vent, im not quite sure on all the details yet because I havnt used it long but both vent and TS3 seem great.
I don't like raidcall much, used it a few times but it just seemed to have less options & crappier sound quality.
Skype does actually have PTT I believe. However it has many downsides as well, it's not really ment as a replacement for gaming VOIP software, its more of a quick video conference or chat rather than open VOIP server.