best addons for a server

Something you can't download a working leak for is VCMod, you'll need to buy it. It's for cars, and makes the driving experience so much better with basically any GMod Car Addon support.
Adding things like ULX and ULXLib is a given, all servers need that.
if you're running DarkRP or a similar RP mod, then adding money printers is a must have, and adding things like a drug crafting system and other things is especially important for keeping players interested.
If you are, then you need things for money management such as ARCBank, though it's hard to get.
Adding shop NPCs and Vending machines and things like that is nice.

Adding things like the M9K weapons pack will fit on almost any server, of any game mode.