I found a new SMP server called Extreme SMP and it is made to be very hard. It is not meant for casual survival and it feels like a real challenge the whole time.

The server has a hardcore economy and the players are competitive so you should expect people to play seriously. Progression is tough and you are supposed to earn everything the hard way.

It runs on version 1.21 and it is for people who only want to join if they can survive.