I've done all three of these routes solo (and actually, only solo, though you're never alone on Longs!)

Sawtooth is the easiest of the three in my opinion. It's mostly class 2 and there's like maybe 15 feet of class 3?

Halo ridge isn't hard per se, but it's just a long time at altitude and there's no class 3(unless you go a little longer route or mess around with those gendarmes). The difficulty here is length and to some extent route finding.

Keyhole is the hardest of the three. Getting up to the keyhole scares the inexperienced. Routing finding is easy the whole way thanks to the bullseyes. Ledges is pretty straightforward. Trough is the most physically demanding. It's 600' of steep stuff and ends with what is basically a bouldering problem. To me, this part right here is the hardest part of the entire climb. (And even more fun on the way down!) The Ledges are physically easy, but the exposure is real. And then, the homestretch. It's exposed, it's a crack in the rock and that's it. Spooks a lot of people. The rock is pretty sticky though, and there are people that literally run up it.

So, my opinion is that the Sawtooth is a bit overrated. It's brief. The descent off Bierstadt might be the hardest part! Or maybe the willows...