Thanks for all the responses. FYI, I've never been over 14,500 feet, but I have climbed Whitney five times, Shasta twice, Elbert, and Langley.
Alaska Mountain Guides seems to have a good reputation. They lead trips to Kilimanjaro, Denali, Aconcagua, etc. I usually try to sign up for one guided trip per year, to learn something about how the experienced climbers handle the details (schedule, pace, gear, etc.).