Psyren
I love this manga. The ending is a tad rushed, but it's still amazing! I wish there was an anime for it, but at least I can read it!
The Principle of a Philosopher by Eternal Fool
I must say, this is probably the most well balanced OP character I've read. He has a reason to be strong, he's lived 5000 years and devoted himself to study, it makes sense that he's OP. Also POCHI IS ADORABLE!! Comedy gold as well.
Hiraheishi wa Kako wo Yumemiru
This one is interesting. MC, John, dies during the battle between the hero and the demon king and is reborn in the past. Determined not to relive the same fate, he starts to make changes with the knowledge he has from the future (namely advances in magic and swordsmanship created during war). I really like that even though John has knowledge of the future his actions are creating obvious changes to the timeline he once knew. He's also not insanely OP and has to struggle at times. Very good, with great character development.
Blood Lad
If you're going to read something today, READ THIS!! It's truly awesome!
Eden's Zero