I don't have experience of all the developers on the list so I can only choose by experience of those titles I have. I have also limited my evaluation to 2018 only and tried to avoid just voting for my "favourite" developer/game.
Reiza - we've had the drift series and the Pumas - can't really say its set my world on fire but I presume they are working on their next title. Not a contender IMO.
Studio 397 - much of the new stuff has passed me by - too expensive so I haven't bought it. But at least the selection of cars in each DLC makes sense and the move to laser scanning is a step in the right direction. Bought the 911 GT3 recently to see if I was missing out. Car graphics do seem better but Silverstone still looks terrible. Why is the new UI taking so long? So overall going in the right direction but not setting new standards.
Sector 3 - WTCR, GT4s and the 934. As usual with RR, the packs are grouped by racing series but none of them appealed enough for me to actually buy them. Same with the tracks out this year.- I like Road America but it seemed too smooth in RR so again I passed. Solid but not groundbreaking in 2018.
SMS - some solid work on patching PC2 (it needed it!) and some great value DLCs. The Le Mans DLC stood out for me - the classic track is a work of art. Mugello in the Ferrari DLC was a great addition - Leipzig in the Porsche DLC not so much. But while we all knew PC2 was not a long term project there is still polishing to do and SMS didn't finish the job. Rules them out for me.
Kunos - superb launch and hype/marketing for their new title - other developers (especially Simbin) this is how it should be done. Incredibly generous EA pricing - for less than the price of a couple of rF2 dlcs you could get the whole game. They have a clear vision and focus for their title which I don't get from many other developers. With lots of pre-release public demos, when the EA roadmap was released it seemed to me they were sandbagging with a very conservative release program. Clearly we would get limited content and functionality but surely what we got would work. Well it seems I was naive. I wasn't expecting the issues with graphics, physics, AI, multiplayer and general game crashing that have occurred. I was frankly shocked at how bad the game looked on my PC and how badly the cars drove, but they have improved with each patch/update.
But despite my issues with the EA version of ACC, for me in 2018 Kunos are moving forward across all aspects of sim racing - physcis, graphics, AI, sound, multiplayer, track modelling, day/night, dynamic track and weather. Combine this with great marketing and generous (almost give-away) pricing, they are my developer of 2018.