Which cassette?
 

Okay, buying a compact double crank 50-34. What rear cassette would give best climbing gear options...........a 12-23, 12-25, 12-27, 11-21, you get the picture.

It will be for a 10 spd. setup.

I like 12-27 cassettes and have them on all 4 of my road bikes (2 doubles & 2 triples). I'm not running compact cranks yet but the terrain around here is gently rolling. I like the wider range on the back that effectively reduces the frequency of shifting up front. I would like to try the smaller chain wheels next time I change something on 1 of the doubles or if I build up another bike. Good Luck!!!!

WHat's your current fitness level?

In general I would recommend an 11-21 with a compact crank.

If you were were running a double before your compact crankset, 53/39, you will get more low end gears than you had before. If you had a triple before, how often were you in the granny gear? Did you use it much? not at all (11-23), all the time (12-27).

A question that only you can answer.

1. What's the steepest hill that you climb? Figure out what gear you need to do that.
2. What's your favorite dead flat road gear? You'll be happier if that falls in the middle of the cassette so that you don't have to make too many front derailleur gear changes.
3. Figure out all of the gear ratios for all of the cassette options that you are considering. The right one will reveal itself to you.

It's obvious. The best climbing gear would be the biggest cog. As to the gear you need, that's a far different question and has to be your call based on terrain and fitness.