I've done some pretty crazy routing in my life. It takes a certain level of creativity - and masochism - to take a non-stop, 5-hour journey and turn it into a multi-stop, 16-hour itinerary (how many segments can you come up with between IAD and LAX?). On the other hand, some routes are already so stupidly painful there is really no need to make it worse. My top 3 most awful routes, in no particular order:
1. Washington, DC to Cape Town, South Africa. It doesn't seem so bad on paper. For starters, South African Airways is a pretty decent airline, and they fly 'direct' IAD to JNB, where there are easy connections to CPT. About 21 hours total, including a 1-hr ground stop in Dakar. What's not on paper: that ground stop often turns into a multi-hour nightmare where you're not allowed to get off the plane even while they fumigate the cabin. I don't know what's worse - the sweltering heat when the a/c is turned off or trying not to breathe in the chemical fog. Oh, and after that delightful ground stop, brace yourself - there are another 8 hours to go to JNB, plus layover, plus the 2-hr flight to CPT.
2. Washington, DC to Singapore. I've flown this route so many times it's amazing I still fly at all. On United, there is a direct IAD-NRT flight, and it takes about 14 hours. Nothing too bad about that, but after 14 hours, even the best lie-flat seats in the world won't keep you from being tired. Problem is, there is still a 4-hour layover, including an extra security screening. Then, just when you think you're about to keel over, you board the NRT-SIN flight... and still have 7 hours to go. By the time you stumble out of that second plane, it's been over 25 hours.
3. Washington, DC to Brisbane, Australia. This is the route I just completed yesterday. It starts with IAD-SFO, roughly 6 hours. After a 3-hour layover, there is the SFO-SYD flight that takes about 15 hours. But wait, not done yet... add a quick layover and the short SYD-BNE flight, and the journey now has taken close to 27 hours. Ouch. Although long and tiring, there's nothing particularly awful about this route. I'm just glad I wasn't heading to Perth.