Not sure if ghost rider could beat carnage, the penance stare wouldn't work on him seeing that the symbiote doesn't have a soul.
Cletus sure as hell does, though.
Against Iron-Man? Tony will win, just because the armor's so damn powerful. It won't keep GR down forever, but GR really can't get him either.
Against Sabertooth? Sabes is stronger, but the Penance Stare could floor him. I'd say GR loses unless he can tag Sabes with the Stare.
Against Deadpool? DP really can't do anything to him.
Hobgoblin? Done before. GR won.
Against 50 Cent? Dawg, Fitty be from the streets, yo, real hardcore. Wassat foo' Ghos' Rida gonna do? It's Fitty.
Basically Venom beat Ghost Rider, when GR tried the penance stare on him he got knocked out because it caused a massive explosion, and carnage is a stronger and more vicious version of Venom, so you make your conclusions.
Btw not sure about the hellfire, it's not actual fire, it's magical in nature so im not sure how it would affect carnage.