We use the "Hi-Jack" and it's been the best $100 I've ever spent on a tool. It'll jack the post out of the ground even if it's got concrete on it (ask me how I know

).
I removed some handicap parking u-channel posts (40-45 of them), and it pulled one of them 40" out of the ground. I don't know who the idiot was that put one in 40" deep, when all the others were 18"-24", but it pulled it out. It pulled out 8 4" x 4" posts with concrete on them. Only issue with the concrete bottomed ones is you need to dig a scoop full of dirt out the top so it's not jacking against itself.
Search for Hi Jack. We got the medium of the line one. Their site links to ebay to get them, but I called them and it was actually cheaper by calling them.
Make sure you get the base too. We don't have that and wish we did (just need to take time to order it).
It'll pull a post in about 60 seconds. Just wrap chain around the post, hook it around the hook, jack away.