There's no way in hell to ever be putting forward one mod of the year, so I can only mention a horribly (and undeservably) small handful of the ones that stuck out to me as my most used content.
LilSki (yet again) - Tracks. Above and beyond the quality of the professional ones we get as default. Finally a gorgeous Glen to throw any type of car around.
Sergio Loro - Tracks. The legendary modder from the GPL era, for dragging his ass out the stone-age to embrace modern sims, and create some beautiful vintage content that is so very lacking nowadays. I've run more laps on his 60s Zandvoort, than anything else this year.
RSS - Cars. I think you all know these guys. Not a lot else needs said. Top quality, best of gear.
Not yet had a chance to properly upgrade my AC with weather, shaders, etc. From what I've seen, there's a ridiculously good amount and quality to add though.