Picking a server is mostly about matching it to how you want to play. A server at the top of a list is not automatically the right one for you - it is just the one with the most votes this month.
Start with the version
Check which Minecraft version the server runs. If you play on the latest release and the server is still on an older one, you will not be able to connect without switching your client profile. Many servers support a range of versions at once.
Then the game mode
Survival, creative, skyblock, prison and mini-games are very different experiences. Decide what you are in the mood for before browsing, otherwise every server looks equally good.
Check the player count
A server with thousands of players online will feel busy and competitive. A smaller one is quieter and often has a tighter community. Neither is better - but an empty server is usually a sign it has been abandoned.
Do not ignore ping
A server hosted far from you will feel laggy no matter how good it is. If a server feels sluggish, try one hosted closer to your region before giving up on the game mode entirely.