Obviously Monero since it's the only coins that force all users use privacy features, they also constantly upgrade their technology from Bulletproof which reduce overall transaction size and Kovri which mask user's IP.
Zcash could be at top if they use zkSTARKs, force everyone to use it and remove/significantly reduce developer reward.
Are there any others that are backed by more than empty promises?
I believe there are none. Most of the altcoins nowadays might have a working product but only a few people use it or none at all.
That depends on the altcoin. Smart-contract and privacy cryptocurrency have bigger user than you think.