I'll go ahead and throw a new name out there: Merchant Plus. While I haven't directly used them, I've worked for 2 companies that have and we've never had a problem. Their gateway (Navigate) is compatible with WHMCS, and they have always been polite and professional in any dealings I've had with them. I'll definitely be giving them a try should I decide to setup a merchant gateway for my WHMCS installation (using PayPal only, right now).
Alex