Crypt, overall is the best. This is because you can easily mass build them, and not have a big decline in experience. A crypt gains experience whenever a shot is fired (even if it ultimately never hits the enemy), so you can have a bunch of crypts fire their 3 shots at a 1 hp enemy and have them all get xp. Temples and dens, on the other hand (unless the den is upgraded to a ballista den, which requires exp), have an instant fire time, so they must have an enemy in range that they can deal damage to for xp to come in. That 1 hp enemy doesn’t give a comparable amount of experience. Also they have really good special effects, including freeze to have a chance for dealing with ninjas. While the other towers are certainly usable, mass crypts is more powerful due to the experience advantage.
The temple gets an honorable mention, because aoe temples are good in some positions (especially high ground) and as an instant mana source.