The battery with the most Reserve Capacity I was able to find was a group 31 battery from VMAX. It has 265 Reserve Minutes (Which I did buy to use as a electronics battery). Was'nt cheap though, but I found nothing over 200 minutes in the same size. My Bluetop Optima I have as a cranking battery has only 120 minutes. Look at my post (Best Dual Purpose Battery) regarding that and it may answer some questions. Good luck.

P.S. VMAX was $349 shipped.