When there's 20+ movies in a franchise, picking a favorite is hard for me. ( Iron Man ) And the MCU is not like the Harry Potter or Lord of the Rings series where it's a straight series, one long story broken into books.
The MCU is franchise inside franchise. Iron Man, Captain America, Thor, Spider-Man, Guardians of the Galaxy, Avengers, ( and the stand alones, Strange/Panther/& soon Widow ) Each series in a series has their Arc, their losers & winners.
In my not so humble opinion none of them outright suck. But today I'll focus on the Thor trilogy.
I don't think we'd have a trilogy without the bigger Universe of films, I doubt Dark World ever would have been made, and if the studio had approved it, it would have probably killed/ended the Thor movies. Lucky for us, it didn't exist in isolation, and we got Ragnarok as a great cap on a trilogy of Family & Growth.
I really liked the first/original Thor film. If it had been the only big Comic book movie instead of a part of a larger project, I think it would be successful in it's own way. I guarantee it would be shown in Film classes as an example of Shakespeare's legacy in cinema, & analyzed to death in campus coffee shops. A cult following would exist, cosplay, cons, fanboys & fangirls writing the studio wanting more...
That the Thor trilogy is the "weakest" of the MCU films isn't so much that they are bad, but that the others were so darn good.
And I really think it's the Curse of the sequel. If #2 isn't fantastic, that can kill the franchise on the spot. Captain America-Winter Soldier was great. Iron Man II was darn good. Thor-Dark World? Ok.
Consider if Aliens had been cheesy like 3. That would have killed it. ( and spared us 3 & the rest, but, hey ) But Aliens ( Alien 2 ) was a great film, a horror-action change up from the horror movie original, but not a copy or reboot in any way.
Anyhoo, enjoy the "why this movie was great" rants in the links above.