How are we defining "best manufacturer of the year"? Based on where you will spend most of your money? Number of new cars (re-)released?
Personally, hard for me to pick. All of them win - or almost all of them.
Slot.it has an impressive line up and it is also branching into GT3. And of course the 512s is going to be a best seller. let's not forget that Slot.it also manages Policar and they have another Rossa coming (the 126C2).
Sideways is another high in my list: their GT3 line up is increasing at the speed of light (for slot car manufacturers anyway).
Revo branching out to small saloon is something to watch...I am still not 100% sold on their noisy gears/metal rattling chassis...but this new class might be something for us digital racers to consider.
SCX: love their WRC cars...pity they go like poo...still love them and I will buy any new releases. I also bought some of their re-releases like the A110.
Carrera will take a huge share of my slotcar budget especially in 1/24. They have also some interesting 1/32 models that I will get.
Thunderslot Ferrari is high on my list. But I only buy whitekits and paint myself. I am not interesting in collecting them. Their move to muscle cars is interesting and if they release enough models this will be another class to consider.
Scaleauto: mainly 1/24. In 1/32 the acura will be high on my list. I still get some of their "old" stock of GT3/GTLM cars when I do not have anything else to buy. They are great performers.
The least interesting:
NSR: I might get their new 720S. Just a white kit. And I still buy some when the price is right. Despite being old and not a performer I like their 997. I also like their Abarth 500 and I wish they did some other small saloon to race against them (Suzuki Swift, Audi A1, Toyota Yaris, etc) But overall this brand does not excite me. They do perform well almost out of the box, but as pointed out they are slottified to the extreme so I would expect so.
Scalex: it is all over the places. There is no clear direction where a specific class goes. Releases (liveries and models) within a class takes ages (BTCC, GT3, Formula E, Trucks, etc.). And they have a tendency of dropping a class because does not sell after releasing a few models (Maserati Trofeo and Mini Cooper F56 come to mind). Overall, it feels like they do not know which direction to go, seems like they are not committed to any class, and prefer a shotgun approach to try to see what stick. High on their list is to avoid to copy other manufacturers' releases but end up doing it anyway (some example AMG GT3, Vantage, C8R). It is a pity because some of their models are very nice and if you get rid of most of their parts they can be made to perform well with a 3dp chassis. I will still buy some odd ones especially on the Gt3 class but it is not high on my list. They have missed a great opportunity by dropping the PCR chassis: that would have been a game changer for creating competitive classes with a chassis that costs few bucks. Sure, you would have needed to add pod, axles, wheels, gears, and motor but for many of us it is a given.