Asus
seriously though, it's up to you. (not that saying that really helps)
MSI has historically had some reliability issues. i had an early Socket A DDR mobo from them, and wasn't too impressed, but then again it wasn't entirely their fault, and they have had a lot of success with NF3/NF4 mobos recently.
DFI has been making a name for itself since the NF2 chipsets as insane overclocking motherboards. Pretty much everyone here (myself included) will make a reccomendation based on personal preference. And mine is for DFI, because i've heard of many fewer complaints about DFI quality.
In the end, look at the feature set, and decide which features you want more. weigh in the price factor, and if you really are going to use DFI's overclocking capabilities. In the end, if one motherboard jumps out at you as the motherboard you want, go with it. If it turns out to be a dud motherboard, RMA it and chalk it up to experience. That's were people develop these opinions anyway.
sorry to get all preachy on you. good luck with your decision.