They all sort of have their strengths and weaknesses.
For parts I would say, Epson, Roland, Mimaki, and Mutoh because most of their machines run on the same Epson print head engine so a lot of the same parts are used on different models. This makes it easier to find, not only OEM parts, but tons of quality aftermarket ones. Can't find a cap top for your ValueJet 1624? Get a Mimaki JV33 one instead. It's like that for a lot of the parts. Also, I've never been trained on a Roland or Epson printer but have fixed many in the field because they are so similar to Mutoh and Mimakis.
As far as techs go, for some reason every independent tech I know works on Rolands. They also usually work on other brands but the one common brand seems to be Roland and I couldn't tell you why. Epson and HP tend to not have as many independent techs but have more official dealer and manufacturer techs so getting help out of warranty or an expensive service contract can be a little more difficult.
One thing I will mention that you didn't is, the availability of the end user to access tech resources i.e getting into service mode to run alignments etc. They all are going the wrong direction in my opinion but some are better than others.
Mimaki - Doesn't give out firmware and software to upload it but if you can get your hands on it (it's on the internet), you can do everything you need to do. There is no password to get into service mode. This makes them number 1 in my book even though they aren't necessarily my favorite brand.
Mutoh - Gives out only the latest firmware and allows you to enter service mode if you know the special code but you do need tech credentials to use the tech software on newer models which is the only reason I put them second. Mutohs are really easy to work on.
Epson and HP - Both are large corporations who have great tech support for models in warranty but once you're out of warranty it gets harder to find the resources needed to do the work. Epson for example has a different software for each model to enter the head rank etc. so you have to find that depending on the model and not one software for all models. HP is sort of special because they have made it easier to fix yourself but when there is a repair the end user can't do, it's super expensive to get a tech in.
Roland - Can't even get into service mode to check your head alignments without a tech login on newer models.