DarkStar LLC for the least lag and best extra features.
I tried out 3 providers, and i must say, Ntaion voice was the worst. They refused to answer my tickets, and that server was by-far the laggiest.
I tried out guild-o-matic, because they also offered a website that was easy to set up. No complaints there, I was able to do everything I wanted.
The third one i tried out was Darkstar llc. I didn't notice any lag, and they offered me a free website if i upgrade to 50 users.
I submitted one support ticket to each, with varying response times
Nationvoice: 96 minutes
guild-o-matic: 22 minutes
darkstarllc: 3 minutes
serioiusly. i was surprised at the difference. What sold me was DarkStar's Clanpay system, which let me pass the bill to my guildies. The ventrilo hosting by darkstar was significantly better than the rest.