Heathrow or Frankfurt airports: Which do you hate more in transit?

Air travel can be a tribulation anywhere but traveling through the world's mega-airports is never high on my list of fun things to do. Last week, I spent some time at Heathrow (in London) and at Frankfurt International airport - two of the world's dozen busiest, and some would say best-avoided, airports.

These temples of transit require travelers to demonstrate the patience of Job, the endurance of an ultra-marathon runner and a good sense of humor to roll with the inevitable hassles. But which airport is best avoided if you are transiting through Europe and have options - Heathrow or Frankfurt Airport?