To build on what Arkhan said:
AT ITS VISCERAL CORE, THE BONK GAMES PROVIDE VERY SATISFYING VIOLENCE: those moments when you perfectly time a head-butt, and then use the resulting momentum to land another well-placed head-butt, and on, and on.
I'd liken the Bonk head-butt to Gogan's (Legendary Axe) fully-powered axe: it just feels so damn sweet when a well-timed swing SMASHES an opponent.
But, with Bonk, it's better than Gogan's axe! There are gymnastics in Bonk (spinning, springing, multiple hits, trying to stay airborne as long as possible) that most games (including Legendary Axe) lack.
Thankfully, there is a rewarding CRUNCH-KRAK-THUNDERCLAP in Bonk that accentuates a solid hit.
...intermission...
QUESTION: Esteban, what are some of the most satisfying moments of violence in Bonk series?
ANSWER: Personally, I love freezing my opponents and then obliterating them with a head-butt. If I can take out multiple enemies without hitting the ground (staying airborne), I have an orgasm. Some boss battles are fantastic because they encourage you to remain airborne by repeatedly headbutting objects (be they projectiles, bubbles, or boss itself).
NOTE: I won't discuss all the other other elements that work in the series (mood, atmosphere, character design, consistency/logic of Bonk Universe, etc.)