Assuming you are not elite on any airline...if so, go Northwest. Exit row seats are almost always available in my experience (connect in Memphis) when you check in. They are not held in reserve for elite members, although those may book them well in advance. Nevertheless, I had no trouble getting these seats 90% of the time when I was a non-elite on NW.

Plus, NW lets exit row folk board with the elites, so you always get the overhead room for your carry-on.

Don't know about all the other airlines, but CO and DL don't have this feature.

Good luck!