Ooooh… Gonna answer this in two ways. 

The most dangerous thing about him that would cause deadly harm to others:

His long memory. Megatron does not let grudges go. He’s gotten better about acting on them except for @slenderwave but he will remember what you did to wrong him and ensure that you end up regretting it in some way.

The thing about him that others often perceive as the most dangerous:

His fighting skills. The mech is a living weapon with all the battles he’s fought and won, even long before the war. His style adapts to suit who he’s fighting, so he uses whatever attack will hit rather than relying solely on his cannon or sword. He could probably kill someone with every part of his body. That’s what people tend to get hung up on.