I´ll go for Toxin.... let me see if i can explain my ideas clearly, here it goes: let's say Spidercide is an "Upgraded" version of Spiderman so as Venom, Carnage is stronger than Venom and Toxin is going to be even more than those 2 togheter. So i guess Toxin gots more to offer and can win this battle.
Well Lobo would win..... im a Symbiote lover but this time, Venoms got a REAL problem. The thing is, lobo is exagerated in all sense, even inmortal (for what appears in the comic vine desciption). You were right about "one-sided"
Magneto dosnt even have to move a finger in this fight.... by the time the Thing finally gets close enough to punch him, Magneto already have launched a whole building upon him. Besides, Magneto can fly