BEST GRINDER????

We have noticed that the quality has degraded pretty much into the disposable category in the last 10-20 years. With Milwaukee and Skil, it's usually the switches that go bad first. We just the "heavy duty" version of DeWalt's 4-1/2 inch grinder about 6 months ago, and so far-so good, but it does not get the level of use that you are describing. I see that Dynabrade is making an electric grinder now, but very pricey. We've had outstanding success with their pneumatic stuff, but have no experience with the electric tools.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

Assuming you want a corded grinder? Uncle Bumblef##@ (AVE) has good things to say about Fein tools grinders.
No experience here, I have a cheap HF side grinder at home.