Depends what you need out of it. CompEx (modules EX01,02,03,04) is the obvious one I'd recommend, having done it myself, but you won't be considered "competent" if you don't already have a recognised electrical installation certificate. There is a design course too which costs more - for some reason I wasn't booked on that - but as a designer myself I felt I learnt a lot doing the course I did. In theory if you're a designer you could do one of those cheap and cheerful ATEX courses if you just want the info and don't care about certification but then it's hard to know if the course is any good.
The Ex 12 design course is reasonable. There isn't anything technically especially challenging per se, but there's an enormous amount crammed in to a few days which makes it tough just due to sheer volume. In my case it is a useful way to demonstrate a level of competence if challenged.
