These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The above mentioned 3-Komas are known as The Big Three. To elaborate: Seiya 3 delivers a wide flurry of punches from over the opponent: hard to dodge and block, and a guaranteed knock-down. Trunks 3 is fast, deals a decent bit of damage, and knocks the enemy away—perfect for ring-outs, as it's also hard to block. Sena 3 is fast (i.e., unblockable), flips your opponent in the opposite direction, and stops them from moving; there are no koma to help prevent movement-seals.
As far as Battle Koma goes, Dio Brando's strongest version is considered to be this, thanks to his useful The World Special, high priority and nicely damaging Rapid-Fire Fisticuffs. Then there's Taikoubou, who is a real bitch to approach thanks to his normals. Then there's his 6 Koma Special 1 which deals MASSIVE damage, and almost always guarantees Ring-outs.
Don't forget Satsuki 2 in Jump Ultimate Stars.
Scrappy Mechanic: For Jump Ultimate Stars guarding is extremely effective against everything that doesn't have the guard break ability since you take no damage from anything, including specials. This can become very frustrating during some of the later missions where AI guards when you attack or just playing competitive multiplayer where your opponent will probably just block and drop something undodgeable on you. To top it off, special attacks that have been known to break solid defences in a character's respective manga doesn't carry the effect over. Very few characters actually have special attack with the guard break quality. Doesn't help that most regular guard breakers are slow enough for your target to stop and sucker punch you.
“Stop Having Fun” Guys (Or Scrub, Depending on who you ask) No Big 3, No Ringouts, No Combo Breaking, Elimination, 90 Seconds, Items and Gimmicks Off, Final Destina- Dark Tournament.
That One Boss: Any character with the ability to auto-guard when not attacking at the cost of SP can constitute. 7 and 8 Koma Naruto is especially guilty of this in JUS thanks to having the ability innately, as well as Tsuna and Robin depending on how good the opposing player is or how unlucky you are.