ABit used to be really good back in the day - boards were reliable but if one went wrong they were fast to sort if
Worst - in my experience (and I've never used ASUS boards) - DFI by a mile -as WJA96 says - the boards are cutting edge to the point of having sharp corners and numerous issues. Worst experience was the botch up that was the S939 CFX3200 board - not stable and a botched RAID controller implementation . Tried to RMA and just gave up in the end
Abit had a class action lawsuit against them over bad capacitors:
You are a member of the proposed Settlement class (the "Class"), if you are within the United States and you purchased one of the following specified Abit Motherboard models during the period January 1, 1999 to the present: BE6, BE6II, BF6, BX-133, KA7, KA7-100, SE6, VH6, VH6II, VH6T, VP6, KT7-RAID, KT7A, KT7A-RAID, VL6, VT6X4, SA6R, AND BX133-RAID. However, the Settlement Class shall not include any persons or entities purchasing Abit Motherboards for resale purposes.
http://ww.xbitlabs.com/news/mainboards/display/20050121031017.html
This was issued in 2005, no doubt it didn't help their financial woes at the time...