I just finished binging One Punch Man Seasons 1 and 2, and I absolutely loved it. But it seems to me that the second season has an incomplete ending, and full of cliffhangers. I want to continue watching the main plot, but there are many confusing titles after the second season, and all of them are named "SPECIALS". So, which one should I continue with?
The specials are set inbetween or after the seasons, so in that sense for something animated it would be advisable to continue with the second season specials, with the chapter
Saitama and Those With Reasonable Abilities
Transcription: Saitama to Sokosoko no Nōryokumono-tachi
but those specials are mostly filler, meaning they add little to the general story and dont resolve cliffhangers. If you want something that resolves cliffhangers you won't find it yet in something animated, and you have to read the manga, or the webcomic, but the manga has become quite different than the webcomic, and it's going to be the manga what probably they use as original source material for the next seasons of the anime.
The reason why it feels incomplete it's because season 2 doesnt cover the whole arc - the arc is pretty long, so the studio decided to end season 2 somewhere after end of the tournament. To be honest, the moment they picked as an ending for season 2 felt very random, probably production was a bit rushed.
Anyway, there's no wa to continue watching the main story. You can try reading manga, but there's two versions: original one by ONE which is very rough in term of art, and Yusuke Murata which is an excellent looking redraw of an original manga, but the arc of Monster Association is still in progress
And as Pablo metnioned, the specials covers fillers that are mostly just silly comedy, but some are showing some backstory of characters like Fubuki or Saitama.