There are a ridiculous amount of temples in Chiang Mai. I don’t think you can walk more than a block or two without stumbling onto a temple. It reminds a bit of Kyoto in that way.
Obviously, I’ve seen quite a few of them. There are a handful that everyone agrees you have to see (like Wat Chiang Man or Wat Chedi Luang), but all of the ones I saw were interesting in their own ways.
Your best bet is to just wander around the city (at least until the heat gets to you), and pop into the temples that you pass by. It’s hard to go wrong.
(Though I will say that you should probably see the two temples I mentioned above — those ones were particularly impressive.)