Disclaimer: It's possible that the damage meter results are skewed by hypothetical bugs, but it's such a simple concept that I doubt it.
Link to the DPS meter: http://www.youtube.com/watch?feature=player_embedded&v=nOLkWKsocR0
Blazing Spiders - USE THESE. At 93k unbuffed DPS, I was doing 250k - 300k damage on average (hitting a peak of 350k), well above their stated damage. For single-target damage these are a great choice.
Leaping Spiders - Also good, but not nearly as good as Blazing.
Well of Souls - Also a good choice, does about 40% more damage than Blazing.* It's not as easy to use though, it uses a lot more mana, and you can't pair it with Manitou for obvious reasons.
Plague Bats - These are just bad. They do less than their stated damage.
Cloud of Bats - This is surprisingly good. It hits packs really hard, but its range really hinders it.
Flaming Darts - If I recall correctly these were not doing as much damage as Splinters, for whatever reason.
Splinters - These did about 200k damage, which sounds reasonable.
Rain of Toads - These did about the same damage as Splinters, which confirms my suspicion that its 1.5x attack speed bonus is legit and does not have reduced damage.
Pyrogeist - Horrible when spammed. Good for damage stacking, I guess.**
*Edited to reflect further WoS testing.
**Edited to be nicer to Pyrogeist