We've been using Digicert for a few years and have been very happy with them. They're on Microsoft's recommended list (especially for Exchange), their prices are reasonable, their support good, and we can regenerate or get new certificates very quickly.http://www.digicert.com/
They're not the cheapest one out there, but their products and service justify the price, IMHO.
We used to use Verisign for everything, but they were slow and it took forever. Not to mention their pricing, as well. We still use them for our primary website certificate (people like to see the Verisign logo), but we use Digicert for all of our internal certificates (wildcard certs are great!), Webmail, Exchange, SSL VPN, etc.