They're all great and have something that the other two don't/can't have. I could go on for hours about each. Also this is a very personal/subjective question, so take with a grain of salt:
Toronto: It's Canada. Culture is different than the US. My friends and family are here. Since it's where I formed my personality, I like the people here way more. It's the only place I experience uncontrollable laughter in reaction to what someone said. The inner city is very cool without feeling like some over-discovered instagram location. In NYC every half decent bar or restaurant is packed with a lineup and costs 2x.
Transit is okay, not great.
But I hate Canadian suburbs, so to me the GTA is not hospitable (for me) outside of the rectangle of Humber river to Don Valley, waterfront to Eglinton. And income to cost of living ratio is so much lower compared to tech in the US, it's like.. minimum 10+ years of working life lower.
Seattle: A true gem of a place. Not overcrowded (yet, despite what people there say). Great music, bars, restaurants etc. You can get from middle of "city" to middle of wilderness in 1.5-2h, and be literally in a national geographic photo. Insane income:cost of living ratio (only slightly lower income than Bay/NYC, but cost of living significantly less).
Transit okay, getting better, but not too useful inside the city.
But it's a very monocultural place. So unless you find your "people" here, it can be quite boring/exclusive. I fraternized widely, but ultimately was left wanting for more socially. Also far-left politics are central to social culture here, so it's quite literally not a "safe space" to be anything but. I'm more of an "east-cost person", if that makes any sense.
NYC: One of a kind place in North America. Feels like you truly live _in_ the city, not just in your house/apartment with necessary excursions. It can be overwhelming with how many people there are and how densely they live, but to me that's something special. Cost of living is insane, but it doesn't buy you nothing. Subway system can feel archaic and let's you down often, but it also enables a special way of life for North America (car-less existence, a fixed 30 or 50 min ride from almost anywhere you'd need to go). A lot of double edged swords. The diversity of people is eye-opening. Toronto can feel more ethnically diverse - which is interesting in it's own way - but NYC is diverse in every other way too.
I'm quite worried about having kids in NYC someday. From what I hear the school system is ruthless. I don't want my kids to undergo such stress. I think Toronto wins out huge for raising children.
Anyway.. bit of a ramble.