I would seek a bike under 20 lbs., with very low gearing, fairly stiff (you don't want frame flex stealing energy), and one that fits me properly and is comfortable. I'd also focus on getting the lightest wheels I could ride comfortably and safely (this will depend on your weight and how stiff the wheels are). I know there are pretty wide boundaries, but that's what I'd look for. I've ridden centuries on two primary bikes over the last five years. The first is a Specialized S-Works Roubaix. The second is a Habanero Team Issue Ti bike. Because I'm not the lightest rider out there, I've opted for Mavic Open Pro wheels on Ultegra hubs. Both bikes come in under 20 lbs. The Roubaix is a bit over 17 lbs. and the Habby is a bit over 18 lbs. The carbon Roubaix climbs a little better than the Habby, but not much. I actually prefer the Habby, because it provides greater comfort for me over the long run.