OK....
My favorite place to spot is at the intersection of the north/south and East/West runways....it is the closest that you can get to aircraft near the airport and have the room to enjoy them. There is a great observation deck there that has an elevated platform so you are seeing and shooting over the fence.
It's a long walk unless you use a shuttle, and heres' how to go about it.... When you step outside the terminal, look for the Holiday Parking Shuttle station. There are 2 stops at Terminal 1 and 1 stop at Terminal 2. With the recent renovations to the airport, they may have moved them since my last time there, so I can't tell you exactly where they are. There will be street signs on the sidewalk for them, or just as the airport info desk. This shuttle is free and takes you to the long term 'holiday' parking lot across the airport. If you leave from Terminal 1, it will go to T2 for a stop and then head to the lot. When you are on the shuttle, pay attention as you near the tunnel that goes beneath the north/south runway. About 100 yards/meters before you enter the tunnel look out the right side of the shuttle and you'll steps that lead to a path just off the road. Thats the path that will take you to the spotting spot. The shuttle will drive through the tunnel (beneath the runway) and pull into the Holiday lot which is just beyond the tunnel to the east of the runway. Get off the shuttle and walk back through the tunnel and to the area that has the steps leading to the path. Take the path towards the runway and you'll see the observation area.
When you're ready to leave, walk back to the Holiday parking lot and get on the Shuttle. By the way, don't volunteer the fact that your using the shuttle to get to the spotting area. I'm not sure how the driver would react if he knew you weren't going to the lot for your car. Sometimes when I was the only guy on the shuttle I'd ask him to drop me off at the steps before the tunnel, and they were OK with that, but don't tempt fate!
What I love about this spot is that Frankfurt sends most of its Asian bound heavies to this runway since it's easier to get the aircraft out of the way. You'll see all the exotics depart this runway if the wind is conducive. You'll see Iran, Kuwait, MEA, etc. Plenty of A380 and 747 action. And whats nice is that it is steady throughout the day. As a bonus, you'll see arrivals from the west or departures to the west since you'll be near the East West runways as well. I've taken thousands of photos from this spot!!
It's the closest place to enjoy the heavies without any fences or crap that can get in your way. There is another spot to the east, but it's nowhere as good as the one I described.
Here are a few shots from my favorite spotting area:
