The options I use are the ones the browsers themselves use:

http://gstatic.com (Chrome uses http://gstatic.com/generate_204 , which returns a 204 No Content)

http://www.msftncsi.com (Windows, desktop and mobile, uses http://www.msftncsi.com/ncsi.txt. NCSI is "Network Connectivity Status Indicator")

http://captive.apple.com (macOS / iOS)

because those are likely to be extraordinarily well-maintained and reliable.

Unfortunately sometimes some captive portals allow just this one site through and not the others. Looking at you, United WiFi, which whitelists gstatic.com, but fortunately not msftncsi.com.