Huh, that's a good question. My views may be skewed, however (as might anyone else's, though), as I don't really know everything about every social group at school. Sports are huge at my school, especially football (though I hear our team is... nothing to write home to Mom about), but I think I'll consider "athletics" as one big elective (oh, I will note that our water polo and boys' volleyball teams are also popular, as well as impressive). Key Club is gigantic! That's definitely up there as well (but there are a ton of clubs (we even have a rugby club!), so it's hard to say). Most impressive, however, is our instrumental groups (bands, orchestra). They've won plenty of awards, traveled all over, and a couple friends of mine have even play at famous places (some Grammy's program, at some place in New York) and with well-known (in the band/orchestra/percussion/etc. world) people.
Personally, I would recommend joining anything, multiple things, everything that interests a person. The school's student body is huge, and the variety of activities is so vast, it's difficult to keep track of what there even is to sign up for in the first place. Our sports teams are phenomenal, we have around or over 30 different clubs, ranging from cultural clubs to bowling, from environmental protection and charity groups to gender and sexuality equality and awareness clubs. Our music programs are top notch, and our art programs are also wide in variety. There's something for everyone, and because of that, no matter what you enjoy, you can find others like yourself, whom you can relate to and have long, emotion-filled conversations with, which I think is the most important part of extracurriculars. Personally, since I am a minority in terms of both gender and sexuality/romanticism, and I think our "small-town, small-minded attitude" could use some human rights awareness, I'd definitely suggest our GSA (Gay-Straight (or, more accurately, LGBTQIA+) Alliance), but really, whatever interests a person is what that person should join.
By the way, whenever I go to websites and they ask for my high school, and there tend to be several Buchanan High Schools. I go to Floyd B. Buchanan High School in Clovis, CA.