The Avengers

X-Men overall, but during their original runs FF for sure.

Avengers, mainly due to my favorite Marvel trio.

Original X-men suck. X-men are something else mostly when they are tweaked in some special lineups. Same with Avengers, although the 4some of IM, Thor, CA and Hulk do make the list most important members, it isn't until they add more interesting characters to actually add an interesting new dynamic since with them today it is a full circle of "been there, done that".

So out of this list, FF is the most legit option. Even though I personally consider them obsolete, they are all irreplaceable within their own ranks.

I monetarily prefer the FF.

FF because they are a family, they were together before they had powers.