Any one of the garage rooftops are good. Terminal A is good for all non-CO ops and activity on 15L. There's a train (located on the basement level) that takes you between terminals. If Jungle Jets and ATRs are your thing, there's COEX's Terminal B. The Terminal C/D garage is a CO fan's dream come true.
If you have a car available, take Will Clayton Parkway east from the terminals. Follow the US 59/Airport Exit signs until you get to Lee Rd. Its easy to miss, but there's a small sign that points to Lee Rd. Southbound. Turn right. The spotter's lot is less than a quarter mile on the left. It's the place to be from 13:00-16:00 as the heavies from Europe come in on 26. If 8 and 9 are in use, its not so good.