Smoked salmon, tinned tuna, and umeboshi (sour plum) Onigiri 
Rice
NikoNiko (cal rose) rice = 3 cups rice : 4 cups water
Formed warm/hot
(Optional) I “cheat” and season like sushi rice with seasoned rice vinegar more often than not. It’s not only not necessary, it’s neither common nor traditional. I still do it.
Tuna
Tinned tuna, drained
Japanese Mayo, or western Mayo + pinch of sugar
Dash of shoyu
Dash of Mirin
Smoked Salmon
Minced/chopped cold smoked salmon (hot smoked is okay, mix like tuna w/mayo)
Wasabi paste, or wasabi mayo
Umeboshi (sour pickled plum)
Whole umeboshi, seeded/seedless (just squish out the seeds with your fingers by pinching), or umeboshi paste
Finishing Touches
I use nori for salmon, black sesame & salt for tuna, and bit’o’pink plum for umeboshi on the tip.
I mix up my filings first, as the rice is cooking… then place my rice in a white wood thingmy ((it has a name, I just never remember what it is (ohitsu), and it’s totally unnecessary. Straight from the pot/rice cooker is fine)).
Fill both halves of the onigiri mold with warm rice, depress a dot with my thumb, add filling, press mold, and add nori/ wakame/ or any other finishing touch so I can tell the suckers apart. If you don’t have a mold? Putting a smallish ball of rice in your damp hands, depressing the center, filling, and adding more rice on top… and then squeeeeeezing your hands together? Also works. It’s just a bit messier. But, very clearly, where the triangle shape first came from! That’s the shape your hands make squeezing the heck outta these things.