First-party support, generous free tier (and cheap enough if you go over 100 submissions in a month), simple implementation, just POST to it like any other form.
A trustworthy choice, though I'm confused what happens after you exceed the 100 subs/month. Their pricing page seems to suggest that you pay more than $19/month since the Pro plan also has the 100 subs limit. Looks like I'm not the only one: https://answers.netlify.com/t/netlify-forms-pricing-model-19...
I also use this. Spam is nearly nonexistent and I can embed analytics in the GET request to the form page (fixes an issue with Google Forms mentioned by another commenter).