Each has its advantages and disadvantages...
Rock Lake - Beautiful lake, incredible views, well off the highway, small very open sites
Pog Lake - Good-sized mostly private sites, waterfront sites are fantastic, on a great lake network, on the old railway (biking) trail, good fishing, close to the highway
Lake of Two Rivers - Oldest campground, right on the highway, good-sized but very open sites under a canopy of trees, next to no foliage between sites, great beach (+dog beach), very close to camp store/ice cream/diner, on bike trail, very popular with families with young kids
Mew Lake - Good-sized, private sites, walking distance to camp store/ice cream/diner, nice beach, quieter than LOTR but still close to highway, on the bike trail
Canisbay Lake - Very large private sites, fair distance from the highway, great canoeing
They're all within a 15 minute drive of each other, so distances from the gate don't really matter much. Personally, I usually go to Pog Lake during the summer (I'm heading up there this Friday for a week). Canisbay is really nice, but I don't get a cell signal there so its pretty much out. Rock Lake is too open for me and its a bit of a long, dusty drive down there. Mew Lake is the only campground open year-round, so I'm usually there in October-November. I'll stay at Tea Lake every now and then (no hydro) for something different - its a great place in the off-season.
Don't forget to stop at the dump station on your way in to fill-up with water. And, make sure to go to the logging museum just inside the East gate. Also, Oxtongue River Provincial Park, just outside the West gate is worth a visit to see the falls.