ME3 is actually the only game in the series I consistently replay without skipping sections of the game I don't like (except the end, it's hard to skip the end). I can attribute this to a number of reasons, and in fact it makes ME3 kind of the "complete package" that makes it, I think, as good as any of the other games. I think anyone who tries to argue ME3 is the worst game in the trilogy is still just pissed about the ending and isn't looking at things objectively.
Now, by the standards of TPSs, ME3 is mediocre but the difference that still makes it fun is Shepard himself. It's not just a "pop up from cover and shoot a guy" kind of game, you're like Superman with a bunch of badass powers. Biotic Charge + Nova combos, Invisible assassinations, lift grenades, biotic/tech combos, it's just fun the various ways you can kill things. That's why, to everyone's surprise, people liked the obviously tacked-on multiplayer. It makes ME3 far more enjoyable from a gameplay standpoint than the previous two.
Aside form that, the character interaction (the main reason anyone plays these games) is at least as good as if not better than ME2 because it wasn't just "go to room, have a chat, leave" and never interact anywhere else or do anything else outside of combat. Conversations and situations with characters feel more natural and organic than in ME1 and 2 where it was this robotic "oh, I did a plot mission, better go around to each room the characters apparently never leave and check for a new convo option". And the dialogue was still well written.
And of course the plot was better than ME2s because a) it actually mattered;

was coherent; and c) wasn't full of holes (until the end). I admit some plot points are contrivances (like Legion's death, the Crucible, Thessia, etc) and some things were not explained as well as they should have been even though clearly there was a reason for them (Legion's death, the Citadel Coup, Cerberus on Sur'Kesh), but overall it was a well-structured plotline. ME2 was propped up solely on character interaction, whereas ME3 has a solid plot as backup.
And the ending? Yeah, it's stupid. But at least it's foreshadowed in Leviathan. Because of Leviathan, people who pick up ME3 today will be far less likely to devolve into a non-sensible rage about the ending and condemn the whole game because of it, they'd just say "Enh, I wouldn't have done it this way" and move on. And honestly, everything that comes before it is so good I can't really bring the whole game down because it had a Gainax Ending. Infact, as people have said, if you cut out the convo with the Catalyst, the game works just fine if you skip to the destroy ending. So really, the ending almost works, sort of, it's just that one-off conversation with the Catalyst that throws everything off. I'm not going to hate the game and never play it again because of that.