Which undergaduate major will best prepare me to become a physical therapist?

You have selected a very exciting field to major. Do some investigation on the programs. I have a friend that is a adjunct professor in our program here. A few years ago we were talking and she informed me that physical therapy programs have become so competitive that it was actually easier to get into medical school. Also, look at the entry level of your program. Years ago, entry for physical therapy was a bachelor's degree. Then programs raised theirs to master degree. Our program here is an earned Ph.D., meaning recipients have gone 4 years for a bachelor degree, 2 years for a master degree and 3-4 more years for a doctorate. Also remember that failure at either level means you basically have degrees with no employable skills; however, you may also minor in a field that will hold job opportunity.

Allied health fields are also wonderful in future job possibility. Occupational therapy, audiology, physical therapy and speech-language pathology are in the top 15 of job needs. Years ago, OTs and PTs were gold until the Balance Budget Amendment passes. PTs and OTs parked themselves in the office or clinic and waited for clients to come to them. After the BBA, they were required to visits doctor's offices in order to drum up business for the clinic. Evaluate the profession and see exactly where you wish to work and then go talk to them. We all love talking about our jobs and how we are there to cure the world.....lol. I initially wanted to become an architect until I discovered it would take me more than 4 years to earn my degree. Then I selected speech pathology, earned two master degrees and beyond. But I am so glad I made the decision.
Good luck. You have the world ahead of you. Find something you love doing and can't wait to go to work each and every morning.

https://www.apta.org/Careers/
This is the link to the website for the American Physical Therapy Association that will take you to information about becoming a physical therapist or physical therapy assistant. Then go to the list of schools with PT programs and call the schools. Also, call the Association and ask many, many questions. Most likely you would want to enter the PT program as a college freshman and complete your studies in that program. It used to be the case that you could study something before entering the program. That is no longer the best way to do it..