Ask every person in the valley about their favorite trail, and you'll likely get a dozen different answers. My answer would probably be the Highline, but that may be a semi-controversial opinion. For many, it's a suffer fest up a steep technical climb out of the Bow Valley with little reward in its descent. However, it's a classic bike trail in the valley and popular with trail riders after some cardio work.
The meat of the trail is a technical ascent with punchy uphill sections that will leave you gasping for air. Once at elevation, the well-maintained singletrack offers some decent flow as it moves up and down the lower slopes of Mount Lawrence Grassi and Ha Ling Peak. It crosses several gullies with great views over the Bow Valley as it moves.
The descent is fast, rocky, and technical, but don't expect many features or jumps. You can climb the trail in any direction, but I suggest climbing from the Three Sisters Neighborhood and descending to Quarry Lake. We'd rate this as a difficult climb and a moderate descent. While I'm not a fat-biker I've heard it can be an epic fat bike ride in the winter.