Heller gets one in the tutorial, when a Brawler jumps him. This is a beast capable of tearing apart tanks and can casually bite a human head off. Heller, pinned beneath this thing and armed only with a knife, kills it with a combination of fury and repeated stabs to the head and neck. There's a reason why Mercer was impressed enough to convert Heller without any further credentials.
The Bio-bomb. Heller's quote the first time he sees it in action just about mimics the reaction of any Prototype fan who saw the trailer
In the preceding fight, if you do the dodges even halfway right, you come off as if (and Heller explicitly notes) that you're running rings around your opponent. Heller's so fast that Mercer can't even keep up with him.
In terms of gameplay, Prototype 2 blows Prototype out of the water. Now you have shields on both arms and you can perform shield bashes, which means you can leave enemies open for a devastating attack. Sometimes it's slicing off their arms, some times it's giving them a German Suplex from 15ft. high. You can consume people in the air now, by slamming them back first into the pavement and crushing them, bouncing back up to continue gliding. It's so amazing.
After consuming a scientist in the Red Zone and getting into a helicopter while disguised as the Dr, an Evolved comes and attacks the helicopter, demanding to know where the scientist is. Heller gladly responds.
Heller: The Doctor is out motherfucker! *kicks Evolved out of the Helicopter*