Alternative Character Interpretation: When you realise Ray is forcibly capturing monsters of near-human level intelligence withoyt consent, and trapping them inside a bottle in card form and only releasing them to fight monsters that will generally kick the ass of said captured monsters (and this is their only freedom)....
However, the sequel series reveals they are actually loyal to him to the point Eleking even performs a Heroic Sacrifice for him.
Crowning Moment of Awesome: Fire Golza vs Gomora and ZAP Spacy. This battles shows just how hardcore Kate's monsters are via the total beatdown of Gomora, and how much stronger they are than Rei's, and also features and epic stand-of at its tail. One of the very best battles in the series.
Gomora's, Eleking's and Littra's battle with Zetton. Specifically Gomora's absolute refusal to give up, seemingly raging against not only the monster and his mistress, but against the convention that the Big Bad must always cripple the hero and leave him for dead. He may have lost, but had Zetton spared an inch, he would've been no more.
Also, the rise of EX Gomora and Reionyx, and the subsequent Curb Stomp Battle the former dishes out.
King Joe Black is a walking CMOA, but his finest moment may be when he takes on and curbstomps all three of Rei's monstes AND ZAP Spacy without so much as a scratch.
And when he deflects Zetton's super-powerful Rechannel Ray with his bare hand/hand cannon.
Also, Red King vs Gomora.
For some fans, the revelation of Kate' third monster- Zetton.
Also, the grand return of Bullton definitely counts from the fandom perspective.
Squick: For all his jerkassery, Bemstar is one of the cutest monsters in the whole show. But hey, what's that stone tablet on his chest. Actually, its a second mouth, which you know, opens to reveal his insides.