Well as the title says, is there any anime that you thought at first "Meh.. guess I will just watch this to pass time." or "Not really interesting but oh well." then ended up liking them after watching the series? Basically anime that you thought wouldn't be as good as it turned out to be.
I do have, let's say 3 of them.
Working!! Series : When I look at the title, and the synopsis, I thought that "Well, it's a good for a time-passer while searching for a good anime." But after I finished the series, including the recently released movie/ova, it was brilliant, one of the top shows in my list if I ever have one. The pacing is really slow, it took 3 seasons just to get stuff going but it's worth it, calling it a masterpiece wouldn't be a long stretch.
Oregairu Series : Okay, my first impression on this was "Oh, a guy in one club with 2 most beautiful girls in the school. Sure, just another anime to add to my list(if I ever have one)." But oh boy, how wrong was I back then, it turned out to be a great series, with an mc that keeps spouting wise word and not-generic heroines. What more do you want? It's perfect.
Koi to Senkyo to Chocolate : So, I actually already said before that this is an underrated shows and that's the truth. People tend to overlook this show because of the possibly another generic harem. I mean really? A guy in one club with 6 girls, this is the type of setting that is destined to be one. But well, tbh, I actually also think that way after watching the 1st episode, but watching it bit by bit, it was my first thought instead that was destined to be wrong. It was great, can't describe it any better, the setting was good, no boring characters, the plot was brilliant, if just the ending went the way I wanted it to be, I would definitely give it a 5/7. No doubt.