Good topic, impossible to answer. There aren't too many ranges I don't like.
Washington's North Cascades have always been high up on my list. Steep and rugged AF, with almost impossible access to a majority of the range. Me gusta!
The Sawtooth Range in Idaho (and the vast mountainous wilderness in general in Idaho) gives it something of a Canadian feel. Not too many places in 'Murka give that true wilderness feeling like Idaho.
Who doesn't love the Sierra Nevada? Huge, high elevation granite slabs that goes on for days, with some of the most impressive trees on the planet on their flanks - not to mention the snow (back in the day of course).
Colorado Rockies are probably my least favorite of the main ranges. While they are impressive (3500M treeline is damn high!!!, some of the highest paved roads in the world, and relatively easy access) they are lacking the glaciated peaks and steep valleys that I really enjoy.
Teton Range. If you gaze upon the Tetons and aren't impressed, there is something wrong with you.
While I have never seen them with my own eyes, I've always wanted to spend time in Utah's Uinta Mountains. They look nice and rugged, yet with really mellow approaches and vast plateau areas. Looks prefect for long summer adventures for getting lost in the wilderness.
Above all else though, the Columbia Mountains and Coast Mountains of BC reign supreme over all else. Some of the most glaciated mountains in the world (non polar regions), some of the highest vertical relief in the world - accompanied by some of the deepest canyons in the world. The snowfall/snowpack, and because it's where I live, my knowledge of the ranges and the comfort I feel when I'm in them. Columbia Mountains are my home, it's where I feel normal.