The first fandom obsession I probably had was Harry Potter, because I used to read a lot of fics and loved the books a lot. Though the one I was really into and would count for this was Smallville, and I still don't know what made my brain click with Clark/Oliver, but something did. I used to google for fics regularly and then I finally had to get over the mental hurdle I had and write it myself and made an LJ account. Smallville was a big fandom, and technically, since I'm still writing for it, I am still in it. But I wouldn't say it's as active. There are definitely people making gifs on Tumblr and it is fun to see new people starting to watch the show via Twitter so obviously I wouldn't say it's dead either. Of course it's hard to get even a fraction of where the peak was with a show that has been done for a decade.
My current fandom "obsession" is probably still the MCU, specifically Bucky/Sam. I don't know if it's a real obsession. Thanks to all the drama from Smallville, I tend to stay on the outskirts of fandom these days, following a select group of people and just reading fic, looking at gifs, etc. They're definitely my comfort ship.
Comparison-wise, they're both obviously big fandoms (MCU in general bigger but SV really had its time) and superhero/comic books based. I am sure both had its share of shipping wars, but the different formats change the way it works. Smallville had LJ, the forums, FFN and with the MCU, it was more AO3, Tumblr, and probably Reddit and Discord. At the very least, overall, there was and is a lot of content, which is nice.