I would guess that no matter which genre of music you prefer, it’s pretty cool when the singer and an instrument “answer” each other, or otherwise directly interact with one another. Sometimes that instrument can be a guitar, piano, or just about anything else imaginable, really.
In the metal genre, one of the coolest sounding examples of this kind of interaction would have to be near the ending of Cemetery Gates by Pantera. Every time I listen to it, the hair stands up on my neck and arms. Phil Anselmo and Dimebag Darrell really shine here.
The entire song is great, of course, but if you’ve never heard this song before and are curious to hear this part, give a listen starting at around 4:50 in the video below. The fun starts at around 4:54.