Wich ones? The most popular is Classic+ made by Russians with lots of new content and changes. Or the upcoming hyped “Project” which is another Classic+ based on TBC talents and system changes.
Or the two vanilla servers? One is in permanent death since Blizzard released Classic (RIP my first server)… The other one is mostly based on Chinese which is on survivable population if you don’t mind that large chunk of the population won’t talk English… And both of them have limited respec costs and few other changes. Chronoboon is even on the second one. Random multiplicative XP events, cash shop with big bags to add to the list.