Proxycheck.io

It detects proxies better than any other service and is officially recognised and used by the Wikimedia Foundation! Because I check for proxies and VPNs for the Wikimedia IPs as a volunteer, but the queries allowed per day in the free plan are clearly too low, I requested a paid status! After verifying my SUL account (the global account of a user that synchronises all their local accounts on all Wikimedia projects), I was granted the highest Pro plan!

PS: An overview of all paid plans can be found here: https://proxycheck.io/pricing