Hyrax in full charge, mouth open!
The one in the entrance of the Smithsonian Natural History museum is spectacular.Hard to say really because I haven't seen every animal possible and so many of the ones I have seen look like old, moth eaten, poorly done, worn out movie prop specimens from the B Westerns. The caveat would be a quality mount! The one that seems obvious but hasn't been mentioned, except as a shoulder mount, would be a full body mount of a large African Bull Elephant. You'd have to have a natural history museum attached to your house but hey the OP's question was, "Which animal makes the most impressive mount?". Good grief.... if they are impressive standing 10-15 yards away in the wild, imagine standing next to one in your special room!
Now from a practical sense, the ones that are a little more common than elephant that I remember looking at and being impressed by include the following examples of large male specimens, correctly full body mounted and in good condition: Northern Gray Wolf, Brown Bear, Lion and Tiger.