If I'm new and was offered Cain, JDS, Overeem, Rumble, I'd be happy to take it. I'd be expected to lose so as long as I can do anything impressive my stock goes up, even if that is just showing "good heart."
Against Hunt, he is coming off a loss and he is 42. Everytime he wins it is considered impressive because of his age. If he lost to a newbie the story would be less "Wow! Look at this newbie!" And more, "Is it time for Hunt to finally hang them up?"
Black Beast is a nightmare. He is on a hot streak, but people aren't certain whether or not he is truly ready for the top guys. If a newbie beats him, it would look great! But unlike the proven great vets, just looking good against him wouldn't be enough to be impressive in and of itself. The newbie has to win for good stock.
But Nelson is the worst first fight. He is the most beatable, but he is getting old, he is losing more, he is gatekeeper status and he is still dangerous. A newbie has to beat him impressively to really get noticed.
So I'd want to avoid him the most.
But I'd really want to avoid all those guys. Id rather fight other beatable no names for my first few fights and hopefully in impressive fashion.
If the question is who would be the scariest to stand across from, all of them! But maybe Rumble would be the most frightening.