And is there a difference for which champions are viable top based on whether or not you are laning with another person, or if you are solo against two? I'll rephrase since that might be a little confusing. Are there champions that become more or less viable based if you and/or the enemy has a laning partner?
I don't understand why Vladimir and Darius are both considered good champions for top when they seem to be polar opposites. It seems to me that good top champions would be ones with range to bully the enemy out of lane, or a champion with an engage like Darius or Xin.
In the current meta, top is always solo, no exceptions...
The standard choices are Shen, Irelia, Malphite, Wukong, Jarmander and Yorick. In ranked, people often counterpick with the likes of Talon, Pantheon, Teemo or Urgot. They're able to build squishier as long as they can still win trades against their opponent.
Jayce is probably the best for a 1v2 top lane (?)...
Uh..... No. Tankier champs with sustain are the best for 1v2 lanes.
No. Champions that are capable of farming safely are the best for 1v2. Sometimes that's through sustain, but not always.
2v1 lanes almost invariably do not follow the 0 CS support meta, which means the lane will almost always be pushed. Thus you also probably want someone who can farm under tower. In addition, someone who can farm at range when the lane is about mid-range away from the tower (as it will be occasionally even with 2 people farming one direction) are a bonus. Conversely, a character with a great escape can compensate for a weakness in the other categories.
Jayce, Gangplank (Parrrley works well in this situation and doesn't have the mana problems it used to), Yorick, Vladimir, Kennen, Nidalee and Tryndamere all come to mind right off the top of my head. There are others as well.