Another vote for the Crooked. It sucks you in with shallow, easy to wade, ankle-deep runs and then bowls you over with inches-thick layers of rock-snot covered substrate. It's so bad, I've slipped on gravel,
The Middle Deschutes can be bad too. Especially the parts where the river is running on the sharp lava. Arguably, the worst part about the Middle D isn't the wading IN the river, it's the wading TO the river. At low water, you have to navigate 100 yards of marshy bull-rushes, beaver dam skeletons, dead-end side channels, and mosquito infested sinkholes JUST to get to the river itself.
I usually wear studded wading boots, so (knock on wood) I've never had much problem.
The lower Selway can be pretty slick, as I learned wet wading with tennis shoes one summer.
It's not just a problem on rivers: There's a section of the Olympic coast North of Cape Johnson/Chilean Memorial which is about 1/2 mile of algae-covered basketball-sized rocks. I met a guy with a bloody nose on that section, and instantly knew what happened. It's certain pain (if not death!) unless you have hiking poles to help keep from totally wiping out.
My vote goes to the M.F. Snoqualmie, or Rock Ck., Mt., but in those places the worst you'll get is wet. In Maryland there's a semi-urban river called the Patapsco., which someone smart once told me is a Susquehanna indian word that means "slow water that smells bad". The human de-comp and huge snapping turtles make it a good time. Cheers!
Yeah, they work for a few days and then the metal wears down too smooth. Certainly better than none, and as cheap as they are, it's easy enough to swap them out. Pre-drill the holes to make it easier on yourself.
The Canyon hole just below Willoughby after the rock garden on the Hoh. Have pulled out and banked on that ledge to the right a few times. Zero vis, and that ledge is covered in algie (sp?). Have slipped off and dunked and have watched several over the years do the same thing. Great hole to fish, but that first hookset literally slides you off the ledge everytime.