I liked both movies but I think Rugrats in Paris was actually an improvement which is rare for any sequel let alone an animated one. Definitely feels to be had when watching those.
In terms of underrated or underappreciated movie...it's tough. I've seen some choices on here where I think the movie gets plenty of praise- American Psycho, for instance, great movie and I think many people acknowledge it as great. But I'm sure there are vocal detractors as there are with virtually any film.
Won't be a popular choice- I know it's pretty widely hated, but that's why I'm including it- I think Anchorman 2 is actually a better sequel than I expected it to be given the long layoff period that is usually a huge red flag. I thought the satire was pretty funny and even though it went to hell in the third act, it was still pretty enjoyable.