After testing campaigns across several industries, I'd say the biggest lesson isn't necessarily choosing the "best" niche—it's understanding the audience within that niche. I've seen people succeed in industries I personally struggled with simply because they knew their customers better. The niche certainly matters, but careful keyword research, compelling ad copy, well-designed landing pages, and continuous optimisation have had a much bigger impact on my results than the industry itself. If I were starting from scratch today, I'd still spend most of my time researching audience behaviour before launching any campaign.