I really like uptimerobot.com -- been using them for years for free and they've got a sensible pro plan.
Maybe you want to check out Up Ninja (upninja.io), it's built by someone from WIP (@obenisik). I don't have used it, so I can't tell you if it fits your usecase, but you can check the response body, also if it is JSON (www.upninja.io/docs?p=monitor…).
Of course @lbckr's hyperping.io/!
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