Boy, that's a good question! I have tried an OCEAN of concealers I think. Here's my favorites:
-Bobbi Brown Corrector/Concealer. Use the peachy paste first to cancel out the purple tone, then the other side (comes in different colors according to your skin color) to blend in with your makeup.
-Dermablend. This is the Granddaddy of all concealers! it covers tattoos, birthmarks, scars...everything, but is kinda heavy for everyday undereye use.
Use a primer over your eyecream before applying concealer. It makes the concealer last longer and go on smoother!
Warm the concealer between your fingers before applying with your brush. It's easier to apply when warm and pliable.
Here's an article I found in ELLE magazine a while ago about concealer:
Undereye concealer should be one shade darker than your complexion, no matter what your skin tone, in order to hide puffiness (which a lighter color emphasizes). However, flawless eyes often require more than one camouflager. (For this reason, Napoleon Perdis [makeup artist] created the three-shade ProPalette concealer.) "To cover undereye shadows without undoing your bag-banishing work, apply a light shade below the swollen area, but above the orbital bone", Perdis says.