For Armour, particularly the Chieftain - Takoms kit is in my opinion the best on the market.
Overall, here are my personal opinions
Armour
Overall - for Armour I would have to say Meng is the best at the moment with Takom coming in a close second. Dragon would be third in my opinion. But there are some great kits out there from others, Tamiyas new kits especially are really well done. I also find that Trumpeters kits are very nice as well - I have not seen a bad one yet. AFV and
Miniart do some fantastic kits as well. Italeri can be hit and miss, same with Academy. Hobby Boss I find surprisingly good kits for the price you can get them at. I also really like the Zvezda tanks - some are really good others not superb but better than many kits out there.
Italeri and
Revell can both be hit and miss in my opinion.
Aircraft
I would agree with the above posts, Eduard really takes aircraft kits to the next level, but most of their kits are actually toolings from other manufacturers with lots of added extras ... so I donot know if they can be classified as a major kit manufacturer. So if you discount eduard I would say
Tamiyas newer aircraft kits are some of the best - especially their new 1/48 F-14 - Really sets a great standard.
Meng I would say is once again on here - they only have I think 5-6 aircrafts but I have a few of them and I can say they are the best I have seen, the only reason I classify them second to Tamiyas newers kits is because they dont have that many kits. Their new 1/48 Mustang which is somewhat snaptite is in my opinion the future of most model kits going forward.
This might be a controversial one here, but once again Trumpeters new Aircraft I find are just superb, their Russian aircraft are all very nicely done.
Dragon doesnt make that many aircraft but theres are some of the best out there.
Airfix's new toolings are some of the best I have seen especially for the price.
I will also say Hobby Boss here as all of the kits I have built (excluding the for children 1/72 ones) are very well done.
Also some of the newer Zvezda kits are superb.
Cars
Well here.. hmmm this is a tough one as I am not a specialist in car modelling though I have built quite a few now.
Once again in my opinion
Tamiya's new tool cars are the best right now - especially the new NSX
Revells new tool cars are probably the second best I have seen with
AMT coming in third.
Aoishima and Fujimi are hit and miss.
For Trucks though Italeris kits are the best out there.
Cars in 1/35 are also being produced more and more and once again I will say the best 1/35 scale cars are by Meng (have a look are their Ford F300)with Takom a close second again.
Bikes
Hands down this category is dominated by
Tamiya - not to say that other manufacturers, Aoshima and some Italeri kits arent that good, just the sheer number of high quality motorcycle kits made by
Tamiya are superb.
Sci Fi
The major manufacturers of Sci Fi kits that I know of are
Bandai,
Revell, Moebius even Zvezda and AMT. I would say if its Star wars or Gundam - Bandai kits are the best out there. If its more movie related Sci Fi Moebius has some superb choices out there. Revells new star wars kits are in my opinion not the best.
Other Categories
I dont have experience with other categories such as ships, railway or figures to say much about them.
The overall best engineered kits on the planet:
I am just adding this in as I feel I need to say this. In my opinon - (and I have built only a few of these) Bandai's Gundam kits are the best engineered kits I have ever seen, its very sad they dont do military or car kits, as they would blow
Tamiya, Meng, Takom, Dragon out of the water. Their parts and fit are much farther ahead of current kits. Also their multi coloured sprues and complete movable kits when finished are just unbelievable. The level of detail on their kits is the best, their fit is the best but what is most important here is that their kits are not that expensive! They can produce much higher quality products for superb prices. I think they can do this because Bandai is one of the biggest companies in Japan so quality is to be expected. Once again sad they dont make regulary military kits.
On a side note Mengs new 1/48 Mustang (and also in some ways Tamiys new 1/48 F-14 Tom cat) has borrowed quite a lot from Bandais engineering and fit which is why I say it is the future of the hobby.
I hope I didnt ramble on too long.... I just had to answer this question!