A newbie shouldn't start with DS9. It's 'Trekkers Trek', as in you probably need to already be a big fan to get into it. Unless the newbie likes Stargate or BSG, etc. An arc-based show won't seem so hard to follow after being familiar with those shows.
TOS, TNG, and VOY are the most 'friendly' shows for casual watchers. You don't really need to know a lot of backstory, or have to follow many arcs. You can pretty much drop in and drop out.
ENT's first two seasons also aren't very arc-based, but then, they are often considered to be the worst two years of the show, so I wouldn't start someone off there. If you have to show someone Enterprise first, show them the stronger seasons: 3 and 4.
TOS, TNG, and VOY are the most 'friendly' shows for casual watchers. You don't really need to know a lot of backstory, or have to follow many arcs. You can pretty much drop in and drop out.
ENT's first two seasons also aren't very arc-based, but then, they are often considered to be the worst two years of the show, so I wouldn't start someone off there. If you have to show someone Enterprise first, show them the stronger seasons: 3 and 4.