What would be the best gearing for mountain climbing?
What parts are you willing to replace? 

You can't get smaller than 34 in the front w/o replacing the crankset with some kind of MTB crank. You'll likely need to replace your front derailleur as well, and there are issues with indexed shifting that you'll need to figure out. 

You can't get larger than at most 32t in the back w/o replacing the rear derailleur with an MTB version. There are also indexing issues between road/MTB derailleurs. If you get a road extended cage or triple rear derailluer, you can probably make an MTB 11-32 cluster work with your current shifters. 

However, in the long run it's probably cheaper just to get a triple chainring on the front.
You'll have a much easier time getting the gears that work for you.