Lately I've been using the soul core of the wall mimics and I really like it. It basically reads "destroy the weak points of all demons and drain all stamina of all enemies in a large area in front of you".
Yes, Nurikabe was my personal favorite Core in the game. Fast, immunity to any and all enemy grapples for the entire duration, dirt cheap per slam, can be wombo combo'd into 3 hits with the 3rd hit doing double damage and being a guaranteed pancake on human enemies, bonus damage reduction as a stat to give a little extra oomph to a tank build, incredible ki damage, and he's practically a Codex mascot
. Only real downside is that enemies tend to sidestep and cause it to miss on later difficulties, so you need to actually time it.
I tried Kinki when the last DLC came out since he has the same damage reduction bonus, but it just isnt the same. Too much movement, you can be grappled, and too much of the damage hinges on the people's elbow at the end.
Ippon-Datara is considered to be the best general use in the entire game since it's so fast you can weave it into combos, also pancakes, ignites in the dark realm, and has fairly impressive damage and ki damage for how spammable it is.
Onryoki is pretty much a must on any build that likes any sort of damage reduction. The passive bonuses are insane, arguably the best in the game except for those builds who avoid damage altogether. Also, the most recent DLC sped up its attack animation (less hits on big shit, but more reliable on everything else) but made it hit VERY hard, and it has a 360 degree attack animation which completely buttfucks Kashin Koji's doppleganger attack.
It's the most expensive core in the game, but Shuten Doji is my 3rd go to core. The attack is fast, anima cost is only 6, but more importantly the core has the strongest defense buff in the game on Amrita Absorption (same power as a Steel Talisman but with virtually 100% uptime) and using the attack gives 15-20s of Extraction Talisman, freeing up even more Onmyo slots.