Usually a dragon is one of the best. Wizards are very powerful as well. Valkyries are somewhat rare, but are good thanks to their spin attacks that can destroy troops very easily. My personal favorite are witches, because they are essentially a permanent skeleton trap. Overall though, almost any troop will work fine, but the ones with high damage and health (pekka, dragon) are generally good in most scenarios. The ones with high damage to space ratios or ones that can fly (wizards, minions, balloons) are better in my opinion, but can be very easy to kill with a lightning spell. I've never had one of the tank units used in a defense yet, but I would think that the lava hound and, to a lesser extent, the golem would be very good, but only if they weren't lured out. I consider witches the best, simply because after a few sets of skeletons have spawned, they start being very space efficient for the amount of troops they produce.
EDIT: I forgot about a few troops. Obviously, wall breakers are the worst, followed by goblins. Barbs and archers are the general filler troops that preform fine for the cost, but don't really stand a chance against their more expensive counterparts (minions, wizards). Healers are good because they can heal troops and buildings, but require units with at least a decent health pool for them to be worth the space. Giants are mediocre at lower levels, but even at high levels when their damage and health are quite good, they are still very easy to lure and kill with only a few wizards unless they are partnered with a healer which boosts their value considerably. Last but not least, hog riders are an interesting unit. They are stuck in the middle ground between tank and damage dealer, but are outclassed because of their stats to space ratio. Valkyries are significantly tankier, deal more damage, have an AoE attack and only require 3 more spaces. This is mostly due to the hog riders ability to jump over walls. Their stats have been weakened to compensate for this extremely useful ability, but on defense all units are given this ability, essentially making their biggest advantage nonexistent.
My Personal Ranking:
- Hog Riders
- Lava Hound
- Giants(without healer)
- Healers(without tanky troops)
- Wall Breakers