Supposing that by "serous" you mean "drama".
I am not someone who is fazed by drama and I find myself rarely ever gripped by those stories (unfortunately my tendency of magnifying details when it comes to "serious entertainment"/drama makes its glaring flaws much more apparent, impossible for me to deem it entertainment worthy of my time).
But I will force myself to pick some...
Dandelion Among Lilies - dealing with a couples drama from a 3rd person's perspective makes it different to the usual stuff, a breath of fresh air to the usual format, so to speak. Beware: Contains many flashbacks.
Octave - A drama with multiple arcs with good writing, pacing, and a natural flow. Also what I found the most noteworthy was the quarreling and arguments and the way the MC handled them was really realistic to the character (as despicable as some might see it), something I rarely ever stumble upon and do appreciate.
...
Now, picking what I consider serious, as in not comedy/silly focus:
Prism - I can't really say anything that has not been said before. Overall superior writing when it comes to yuri... Also by proxy Stretch even tho it's not yuri (but teasing it enough to overload and kill the readers) the writing actually becomes better on just about every aspect.
Dang, I shoulda just popped links to the mangas or something, typing all this instead of reading hentai.
Also, this is the best ever /trolololol