Re: Reaching High Levels
Baal runs is probably the fastest. If you have a cooperating team then you can unparty and get the killing blows on the bosses to get more XP. Some high levels do this anyway despite whether other are cooperating - it's referred to as topping.
My highest is lvl98 in 1.13. On previous ladder characters I've reached lvl96 and lvl97. It is a mind numbing grind going for lvl99 which is why almost all of the ones you see on the ladder are botted.
To give you an idea of the effort required, this is what I found for my own play style. Playing for 3-5 hours a day, at a rate of 5 minute baal runs (12 per hour) I obtained the following track record. There are many variations because you can't always find 8 player games, run times can be slow depending on the runner, you might have topper's taking your XP, you might miss joining a Baal run, you might get a ban, you might take a day off, you character might die (mostly from boredom) etc.
95->96: 8 days
96->97: 21 days
97->98: 45 days
98->99: expected to be 97 days but I gave up as soon as I hit lvl98.
As rule of thumb, once you reach the 90's it takes roughly as much time to get to the next level as it took to get from lvl01 to your current level. In other words once you reach lvl98, you are only half way there.