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Welcome to FT!
If you Google "BWI airport wiki", you'll find a link to BWI's Wikipedia article, which should list all of BWI's current carriers and their nonstop destinations.
You really need to prioritize what is most important to you from an FFP: lounge access? upgrades? free flights (and if so, to where and in what class of service)? Are you willing to connect somewhere to earn miles in your preferred program, instead of taking a nonstop on a different carrier? In what class of service will you be flying when traveling for work? Does your employer restrict your choice of carriers?
Recommendations will differ, depending on your answers to questions like these.