My greatest regret is non-academic: I wish I'd got a classic fast food or supermarket job. It's all worked out just fine, as I've had multiple jobs since, but I think I would've grown faster as a person in the ways that actually matter (basic soft skills, communication skills, etc), which I was really lacking in at the time. And still have definite room to grow.
Academically, doing English Language rather than mainstream English, and doing a 3/4 (6th subject) in year 11, would have been the biggest contributors. Not a huge deal though; my score doesn't matter, and it was probably genuinely good for me to do a subject that caused me to panic, as I was used to cruising easily through school.