I think it's kind of disingenuous to compare the format/style/polished nature of animation spanning a time period of 30 or so years. If G4 doesn't have the most polished animation, given the advances in tech since the 1980s, then it's failed in its objective.
I feel like to answer this properly means to strip the technical side of G4's animation style out of the equation and just talk about the art design..? I mean, G1 animation has a lot of bloopers, but that's also something common to most other eighties shows.
I don't know if I'm explaining it right but the "G4 animation is best because it's more modern" angle sounds like something the other side of ponyland would come up with.
I honestly am not surprised to see people say RaMC. Weirdly for me as a kid it took some getting used to. I actually, possibly like Catrina better, but I can't tell you specifically why that is. But with time I have got to like the RaMC animation style as well. I grew up with only 4 eps and they were all from MLP&Friends so that was what I was used to. I think that's a factor in how I think of these.
I am not sure about the style of Tales animation, although I love Tales possibly most of the G1 animations. I think my love of it came from the fact it wasn't full of what as a child I thought were 'mistakes' (ponies not sold here/featured under different names) or the fact I first saw it at 13 when I was just about turning teenage. It's not the prettiest animation but I guess I have fondness for it.
Some G3 animation is quite beautiful, some of it is a bit odd.
Gonna step away from the shows for a bit, though, because my favourite animation is probably animation that appears in commercials. Or that rainbow snippet we haven't quite identified yet. In terms of commercials, even G2 can be included in the discussion. I remember the G2 commercials when they were on TV, so I suppose I have a kind of fondness for them (and the 'animation?' in the game I guess).
I don't really care for FIM. The animation style is ok, but it feels like it's trying to be...something. I don't know what, but it's especially how they animate Pinkie when she's having one of her silly moments. Like it's trying really hard to be cool and funny and...eh, it's not my thing.
The summary - I am not huge on the animation, that used in some of the commercials is interesting and different, and probably like Catrina's animation more than RaMC if you have to push me. And let's not leave G2 out - it didn't have a series, but it did have animation.