Know thyself.
In other words, what suits you best?
Full time or distance learning?
Smaller classes or big ones?
Smaller establishments or bigger more regimented ones?
I think you get my drift.
For me, I wanted a full time course that had a good reputation and represtented good value for money and wasn't part of a huge establishment. So, I picked ATA at Coventry. Self study wouldn't have worked for me. Too many life distractions
On the other had, Checkers wanted just to do some brush up courses that would fine tune his exam technique. He picked Bristol Ground School. He reckoned BGS was one of the best theory providers he's ever seen. (Former Ansett 737 pilot)
We both passed our ATPL exams first time.