They're all different but the server list lets you sort by player count, so it's fairly simple to find people and avoid empty servers. I highly recommend trying 5-10 different ones before you settle on a server, since they can be so vastly different from one another in both content and philosophy.

The ones I enjoyed the most when I played were Cormyr and the Dalelands and ALFA: Sea of Swords.

Cormyr and the Dalelands has a large player base, a wealth of awesome static and custom content if you prefer solo play, and frequent DM events. This would be a great place to begin.

ALFA: Sea of Swords is a small server that is hardcore rules, low magic, perma-death, and more of a close knit group of players. The server's great but the player base is small. It's hard to play on this one because you basically have to join the discord, get to know people, and coordinate partying offline. I still think it's worth it though, but since your character can die permanently there's an extra thrill to the dungeon crawls.

I would avoid Arelith even though it's one of the most populous as I had some bad experiences with both the players and the staff there. It's a little griefy.