Generations was the first EP I bought, at the same time as I bought the base game. It's also the one I miss the most if I play on my laptop with no EP's. But I have completely eliminated the IF's with Retuner, and with my sims on low free will, my child sims don't ever preside over the royal court unless I decide they will, and that doesn't happen very often. But whenever possible, they always get to play submarine adventure in the bathtub.
Regarding the three you are considering, that's a tough one. I like all of them. But I guess Seasons is probably my favorite of the three, mainly for all the new recreational equipment and other activities they can do, like making snowmen, snow angels, and igloos, raking leaves in the fall, water balloon and snowball fights. I like anything that gives my sims new activities they can do.