Mule deer, Columbia blacktail, Sitka blacktail here. Only whitetail I've ever hunted was the Coues in the Southwest when I was growing up. Never got around to hunting Eastern whitetail, but spent a lot of time around them.
Toughest to hunt by far was the Coues, even with modern rifles. A guy might put in years to take one with a muzzleloader. Nearly as tough is a trophy Columbia blacktail. They can live in the thick stuff as well as any Eastern whitetail, but they don't "pattern" worth a dang with their huge territories and random habits. Worse yet for trophy score watchers, a 3x4 is "typical" for big bucks, and that fourth point on a 4x4 is usually nowhere near the size of its mate on the other side. Lotta deductions when scoring most, no matter how long the points and how wide the spread.
Sitka blacktail share the "point" issues for trophy hunters. The love the deep tangles too, but frequently venture into open country you'd expect for mule deer, as well as the high stuff you'd expect for Dall sheep or mountain goats. Add in tough weather and big bears, and they're my favorites to hunt, if not nearly so tough as Columbia blacktail and Coues whitetail.