1.) Which VOIP software do you prefer?
2.) Please explain your answer to question 1.
Mumble has a better VIOP protocol with a lower impact on my overall latency (virtually none) as well as maintaining better audio quality (at least for those with decent mics) and the client is not bloated like the others.
It's FOSS aspect with the different levels of customization are also very nice. Likewise, Mumble has the best native Linux client available in terms of features and responsiveness.
3.) Do you have personal experience with all three choices?
I've used TeamSpeak since 1.5 and Vent since almost it's inception; Vent was my previous "favorite" but only because it was the "less of two evils" so to speak. I often would just use in-game VoIP if available, as both programs have had dodgy quality for me over the years.