A level, but eh...
AQA - best format imo and best resources, especially for geography and biology. Sometimes, their wording is a bit... off.
Edexcel - they're OK, but not a huge fan of chemistry. The format of the papers is good, but there seems to be a slight lack of resources (like no official textbook for the course? Never got given one).
OCR - the worst exam board. Had them for maths and the paper was weird. Looks fairly old style too.
I did have WJEC for GCSE English, but don't really have an opinion.