I am currently a senior in high school and have a question about majors. If you apply to colleges, would it be best to apply as undecided? I’m aware that admissions to certain schools are very competitive no matter what major you choose, but will the competition be diminished if you choose undecided as your major? Thanks!
Nope–apply with a major. It makes you make sense as an applicant
If you have a clear idea of a major, apply for it. It is sometimes difficult to transfer into a major once you are on campus if the major is popular and filled. If you truly are undecided, then you should choose undecided.
If you have an intended major then apply for it since it can be difficult to switch once admitted to the more competitive majors. If you are undecided then choose undecided and maybe focus your list on colleges that have flexibility on getting into all majors.
there is no blanket answer. Some schools don’t consider major at all, some only do for majors like theatre or engineering, some admit by major. You’ll have to look at the admissions websites of the schools you are considering to find out.
On common app you’re better off choosing a major that makes sense with your hs curriculum choices, and is less requested. It won’t make a huge difference though.
At universities that requir out to apply to a major, apply to the major you are interested in. The odds of an internal transfer at these universities are very low.
Thanks so much for the answers so far! I have an ideal major for each school I want to apply to so far, as well as a back up. 
The vast majority of college and college admissions professionals are not expecting you to have it all figured out from age 13 - to somehow plot and plan your activities so that they seem to follow naturally from the time you were a freshman in college to the time you pick a major. College admissions professionals (and most adults) understand that high school students don’t necessarily know what they want to do with their lives - or even just in college - at age 14 or 15, and understand that you might change your mind considerably.
Therefore, you don’t have to pick a major that makes you look like “make sense” as an applicant. It’s fine to apply as undecided. It’s also fine to apply as a political science major if you have done a bunch of environmental science projects, or as a computer science major if you’ve done Model UN and debate team, or as a philosophy major if you’ve mostly done creative writing.
The few exceptions would be:
-At colleges where majors are competitive/impacted, it might give you a competitive advantage to have some prior experience. This is particularly true in engineering and computer science, where increasingly you have to apply specifically to those majors. But that’s only true at some schools - at many colleges, you can major in CS without applying specifically to the major.
-Certain majors require portfolios or auditions at most colleges. Those are mostly fine arts majors: visual art (including film) and performing arts like acting, drama/theater, musical theater, instrumental or vocal music performance, and dance, as well as fashion in some cases. In those cases, you need to have some exposure/.experience to build that portfolio or build the skills necessary to audition.
But even that’s not universally true - at many colleges you can major in drama or dance or music or art without auditioning and a portfolio, especially at many liberal arts colleges.