What's the best caulk type for exterior stucco to wood window trim?
In my opinion the best caulk for almost any siding job is urethane. It's crazy sticky, paints well (and comes in a zillion colors to match pre-finished siding by special order), and remains flexible. 

Caveat: As I said, it's crazy sticky. It will remain on your skin for days and your clothing forever. You'll want to use a small, well-shaped nozzle and practice beforehand. Ideally, there's no additional tooling to be done. The nozzle does the tooling.