Kraven & The Lizard, but add in Chameleon.
Chameleon: He the instigator of the feud between The Lizard and Kraven, if he hadn't been so greedy and self-righteous, he wouldn't have gotten killed. His motives were all a mess after it all came full circle that his plan was to prove to people using Dr. Connors that the formula worked as his main plan was to sell them fake copies for a high amount of money. Chameleon/Dmitri in the fic is a schemer and he's the main cause of what happens during the final few acts.
The Lizard: Just introduced The Lizard and he killed Chameleon in the storyline so far, you see how he leaves his son behind that he's truly lost inside this monster. And if you ever wanted to get Dr. Connors back, you'd have to work real hard to find a way to get him back. The Lizard is pretty much the Dr. Jeykll & Mr. Hyde aspect of the comic-books, but in this he's just a full-fledged monster. This is one of the reasons why The Lizard can't work as a main villain in any of the movies, you'd have to work super hard at getting the character of Dr. Connors to his full-potential in order to create a great connection between the villain and yourself.
Kraven: Kraven, as you know, is Dmitri's half-brother. It was my idea to first introduce Kraven before Chameleon's death but I found that'd be too cliched if you called your brother up to deal with a giant-lizard, I didn't want that. So I decided to kill off the character at the hands of The Lizard, get someone to inform Kraven of his brother's demise so you can create more tension between Kraven and The Lizard, with Spider-Man caught in the middle.