Minecraft wiki entry for Multiplayer_PvP_bases, associated text: If you're in the ocean, you should usually keep going until you find land. However, if you already have everything it might be better to make an underwater base in the middle of the ocean (discussed later).
Strongholds can be used as good bases. They allow for potentially exclusive (if no one has found the other two strongholds) and easy access to the End; they contain plenty of books for enchanting, and your base will be already built and most likely secluded without you even having to lift a finger. However, they are somewhat easy to find for any player with eyes of ender.
If the current biome you are in does not fit to your tastes, you can build a base in the Nether. Make sure to build it a decent distance away from the entrance Nether portal to prevent it being easily found by others.
Check in on what plugins are in use on the server by doing /pl. If you do not have permission, ask a high ranking staff member or the owner. Different mechanics, such as a guns plugin or a steel mod, can greatly change the worth of resources and you should double-check it before settling on a place.