Venom vs Carnage
As his first appearance, the Venom vs Carnage arc would be the most important collection of issues to have for Toxin. He is spawned from Carnage in the story, so is essentially born which would seem like a pretty large part of his story. It can be collected in a trade paperback of the same name.
Taking place after the Venom vs Carnage series, this miniseries follows Pat Mulligan as he tries to use the symbiote to capture supervillains who escaped during a breakout at the Raft prison. It can be collected in Toxin: The Devil You Know.
Toxin makes a number of appearances in the Venom series although he is bonded to Eddie Brock rather than Pat Mulligan. Venom: The Savage Six is his first major appearance but he does show up throughout a number of later issues as well.