Based on personal experience, my vote would be for the 6.5x55 SE in a rifle that wasn't really light in weight. Using that as a familiar starting point, I would feel comfortable recommending cartridges with similar statistics (e.g. the 6.5 Creedmoor). The best thing about making a decision like this is you're going to have a variety of options to choose from. Since avoiding recoil is the goal, I think increased rifle weight & the selection of a quality bullet (possibly a custom reload) would be worth considering as much as the caliber. Using a semi-auto (include AR-style rifles in larger calibers), suppressing it, installing a mercury recoil suppressor in the stock, adding a quality recoil pad, etc. might all be viable options to reduce felt recoil.
