What are the best mosquito-controlling critters for a pond?

White Cloud Mountain minnows are extremely effective at controlling mosquito larvae and are hardy too.

Any small fish except possibly goldfish/Koi. My Koi are confirmed vegetarians, never make any attempt to eat fish of any size. Guppies and gambusia are also called "mosquito fish" because of their appetite for larvae. I would prefer livebearers, anyone will do: swordtails, platys, mollies, whatever is on sale. These fish like warmer temperatures, but could be put in at 70 °F (21 °C) water temperature and would reproduce and eat larvae until the water temperature drops to about 50 °F (10 °C). They will tolerate this lower temperatures when living in a pond. Not nearly as interesting control of mosquitoes is adding Bt (Bacillus thuringiensis) to the pond. It will kill any insects and not affect fish, amphibians, reptiles, or mammals.

As far as I know, dragon-fly larvae feed with mosquito larvae, and dragon-fly adults feed with adult mosquitoes.

Now, where do you get the dragon-flies from, is another question, which I cannot answer. Also, I cannot tell if you enjoy seeing dragon-flies around. Personally, I am kind of fascinated by them, I would have no problem having them in my pond- especially if they control the mosquito population as well.