re: What'll be the 1st MCU movie to be scored rotten/1st DCU movie to score fresh?

I expect Suicide Squad will be rated "fresh"; it looks like it will be a lot of fun. Doesn't seem to have that grandiose melodrama feel that a lot of critics attached to BvS (the Watchmen comparison);
plus, if it's even decent, I think the critics will be careful not to appear to have a vendetta against DC movies. BvS has made a decent amount of money, and there was a wide disconnect between the audiences and critics- the audiences felt it was ok but not great, while the critics tore into it.

Marvel "rotten", I would think either Thor or GoTG 2 would be at most risk. Although I love both properties so far, I know Thor seems to be the most vulnerable to complaints. Might be the "Superman" factor, it's pretty tough to put Thor in actual danger, and it seems like Loki is the prevailing reason for good reviews.

Guardians, I think, was a big surprise that took advantage of the scifi aspects and the nice casting chemistry. Both of these could be dinged in a sequel:
Star Wars is going to affect how scifi action is viewed by critics, and all the quirky dialog humor that was so fun the first time, might come across as forced the second time.