Depends on what era you want to start with.
I'm not very good at the original run and it's kind of confusing to be honest.
Reboot isn't really collected but lots of fun. It's a slightly more modern take on a young Legion and the mood and cast tends to switch around a lot. You can start with Legionnaires vol 1 or The Beginning of Tomorrow (Or Legion of Super-Heroes vol. 4 #00, if you want to read issues)
The Threeboot is more dark and edgy than the first two runs and a lot of people aren't super fond of it, but it's fully trade collected, fairly independent and very easy to follow. You can start with Teenage Revolution (or Legion of Super-Heroes vol. 5 #1)
The Retroboot starts out pretty good but kind of got fucked over post-FP so you should stop reding right around then if you choose this one. It resurrects the first run and modernises it a little, showing an older team that deals with more serious issues. Its beginning is a little over the place but it's easiest to start with The Choice (Or Legion of Super-Heroes vol. 6 #1)