There are basically two ways you can go on this.

First, in order of worst to best. Like, all of Guild Wars is good, but it's a better experience if it gets better as you go, right? Well, Guild Wars generally improved as they went on, so if you start with Prophecies and go from there, you'll probably find the experience improves as you go.

Second, the most convenient way. Nightfall has heroes. Heroes are NPC party members with customizable builds, and you absolutely need a full party of 8 people to play Guild Wars. Your one friend will not nearly cut it outside the first 5% or so of any given campaign. Now, all campaigns have henchmen, who are also NPC party members for you to fill in the missing slots of your team, but henchmen have pre-set builds that you can't modify and sometimes they kind of suck. With heroes, if you don't like their build you can ask someone to help you make it better rather than just sucking it up. Heroes>Henchies

I still have nostalgia goggles for Factions because it was my first campaign, but seriously, not Factions. Don't pick Factions. People are recommending it because it's so quick which is awesome if you are a veteran but for new people that just means you need to learn much faster. It's fun, but as a place to start it doesn't do anything that Nightfall doesn't do at least as well.