the most important addons for online gaming
I want few most important/common ones since there are many and some are big to download.
and where to download them?
It really depends on where you're playing. Most servers won't really even accept addOns in fear of cheats.
Most important are depending on the server you want to play on as Rellikki said, a server that uses addons/mods often do not use the same ones as another server.
So look into the OFP MP internet browser , you'll see usually the name of the misison played and under it you'll see the mod folders used by the server.
When no mod folders are used, you'll see only RES listed under the mission name
Just look around to get those to join this specific server.
Very often servers make their own addon packages, so you'll want to give a look to the clan/squad/server homepage to get their packs instead of getting their needed addons one by one.