I’d vote for Mexico every time over the others. Id vote for Guatemala next.
Montreal, is nice, fine for a weekend, not much more. I’m sure you’ll enjoy it but to me, it doesn’t give you a real sense of being in a different culture.
Guatemala is really interesting but if your plan is to only see Antigua and Lake Atitlan, you are really missing out by not seeing Tikal. If you want to feel that you are living in an Indiana Jones movie, go there for sure. You can take a short flight from Tikal to Antigua I think, and I would recommend that because it’s a long, hairy bus ride otherwise.
Mexico has so much more to offer than either of the others, IMO. It’s a big country though. I personally love Mexcio City and think it’s one of the best cities I have ever been to. I can’t comment on your other destinations, but I’ve heard good things about them and I am 100% sure there will not be herds of tourists. In the area your consideration, I think this place is a must. It looks fabulous. 2025 Full Day Tour in Tolantongo from San Miguel Allende (San Miguel de Allende)
I highly recommend Merida and the Yucatan peninsula if you haven’t formally decided.
I seriously doubt you will find swarms of tourists in either Guatemala or Mexico. However, it will be hot. Guatemala was okay in the heat though. We went in the middle of summer and survived just fine.