They vary a lot in shape/size/stiffness, so I wouldn't just go with the cheapest. I think the reliability really depends on how it fits your particular body.

I had a Diva Cup #1 for a while but it kinda leaked and made my cramps worse so I stopped using it. Then I got a MeLuna medium sport (which means it's a bit stiffer so it's less likely to leak). That one didn't leak but still made my cramps worse and I could feel it hitting my cervix, so it was probably too long. I know I should just get a right-sized one but have been procrastinating and using tampons.

In any case, I found this kinda fun quiz to help you figure out which one to get: https://putacupinit.com/quiz/

And their incredibly long list of about a bazillion different kinds of cups and their sizes: https://putacupinit.com/metricchart