I'm not surprised, back when I was using them at uni they turned off my mobile data without warning.
Called them multiple times, no-one would tell me why. Went to a Three shop and they told me I'd been disconncted for "security" because I'd been away from the address on my account too long!
Plus their broken app and abysmal coverage in London...
It's not a case of "who is the best" but rather "who is currently the least worst"