Looking for cool mods

Well depends on what type of server you plan on making (such as)
Creative, Survival (like RPG, Factions, Skyblock, Clans based, Role-play)
Then focus on what you want to include, I mean so many mods out their that do so much
custom code / and give allot of aspect to a server.
Using Forge to implement these mods based on version of the mod, and making them all work together without conflict. Total Enjoyment
Tekkit: Was my favorite Mods for a longtime, but updates to newest version of Minecraft for it, has halted allot of play for me. (not Supported yet)

Anyways, Best of luck on your server!