Normally I'm a big proponent of OpenVPN/OpenVPN-AS, but if you're looking for an iPhone client you're out of luck, unless your road warriors are willing to jailbreak. If you don't want to install any extra software, PPTP is probably the easiest thing to set up on both ends.
EDIT: Heh, if you're not worried about the wrath of the iPhone user, OpenVPN is great. I'd recommend OpenVPN-AS if you can afford the pretty cheap ($5 per concurrent VPN user!) cost, as it greatly simplifies installation, configuration, and management. Have had very good experience with it on Debian/Ubuntu hosts.
If you absolutely don't want users to have to install clients, PPTP on debian is also pretty easy to set up. If you want Windows users to use the VPN server's gateway for internet access, make sure that they check the 'Use default gateway on remote network' box under [VPN Connection Properties] > Networking tab > highlight Internet Protocol(TCP/IP) in list and choose 'Properties' button > 'Advanced' button (this should be checked by default).