What I did was just looked through a bunch, found one that fit my needs with the shortest waiting time that I felt was okay to deal with.
One thing to note though is that waiting time only starts when they respond to you. It took like 6 months for then to acknowledge my referral for my 6 month (or whatever it is) waiting time to start. I've never seen any state how long it takes for then to process referrals, so that one is hard to guage.
I went with ADHD 360 if that's worth anything. But also I've not seen them yet either so ultimately don't now if they were the right choice yet.
Things to look for is like if you're wanting medication, make sure that they do prescriptions. If you want counselling, make sure they do that. And also look into any costs that may incur for any of those beyond assessment and that you can afford them.