I think Brent's Data is the most believable "robot" character that's ever been seen on film.
You watch TNG and it's like - yep he's an android - not an actor in make up.
I completely agree. There was an entire section in my previous post about how I can watch TNG and not even see Brent Spiner, just Data the android, but I cut it out because it seemed to maybe contradict what I was saying about admiring Spiner's amazing performances. I just left in the part about enjoying it on both levels.
Problem, for me, was when he was asked to act different characters. While he had honed the role of Data to a T over the years, many of his other characters were more a mockery than anything else.
I liked his mad scientist in Independence Day, although some might argue that that was a caricature, which supports your point. But he played it well. He also did some good one-off characters guest starring on things like Leverage and Warehouse 13. But I've seen some pretty over-the-top characters from him as well, so I see where you're coming from.
Over the years I have concluded that Brent Spiner is actually a pretty bad actor.
I think it depends upon the role and how seriously he takes it. Also, some directors may hire him and expect a certain type of thing, so it's not always his fault. Having seen him do at least a few things other than Data that I thought were pretty good, I can't agree.
My problem with Brent is that, after seeing and hearing all of his Data and Lore voices, pretty much anything he says or does reminds me of one of them. That's isn't necessarily his fault either, though. Data and Lore had a huge range between them, and there's only so much Brent can do to change his voice. So maybe "somewhat limited" applies, but I don't consider him a bad actor.