Urban Assault/YMMV

Scrappy Level: Any corridor mission. To elaborate, corridor missions have a battlefield that is very narrow width-wise, but stretch on for MANY sectors length-wise. Your power station's output will always be less than optimal, so you'll only be able to build units at a snail's pace. The AI suffers from this as well, but since they start with the whole map under their control and multiple power stations, it's like trying to swim upsteam, except instead of water, there's 100 enemy units gunning for you. Enjoy watching your squadron (that it took you 30 minutes getting the energy to make) be whittled away by the constant barrage of enemy units. Rage Quit is very likely to occur.
That One Level: Hamburger Hill, named because the hill in question is flanked on both sides by multiple enemy host stations, resulting in getting attacked from multiple directions simultaneously.