Best Gmod Addons?

Pointshop 2 for sure

Fas 2 for weapons its better for roleplay

ATM - VCMOD - M9K - Anti Cheatcake - And so more...
You whant who gamemode ?

customizable weapons.

Depends on what you're looking for. I'm more into the utility add-ons. Neutron Admin Panel is badass, I would highly recommend.

Wiremod
https://github.com/wiremod/wire

M9K Weapons or FA:S2 Weapons or whatever it is called are really good weapon addons, Car addons are also great

All the Star Wars addons !

Im a TTT guy so the gravity grenade is pretty cool

Gmod legs 100% it's awesome.