A good caster should be, in order of increasing importance.
A) Good at the game, or at least passable. A good game sense will help them prioritize on what to comment on. They should know a wide variety of BOs for every race, and have executed them during actual play. "This is the most normal thing in the world." Husky and HD, and a few others fail on this.
B) Knowledgeable about the game. What counters what, what properties units have, names of upgrades and tech and how much things cost. When a cast is going on they should be able to point out little things like "Hellions do splash in a line away from them, so against large balls of melee units it is more important to minimize distance, rather than attempt to kite", "Ghosts are actually not considered light for gameplay purposes, so the only unit that does bonus damage to them are Archons", "The Corruptor's bonus damage is only applied to massive units like the Broodlord, Carrier, Battlecruiser, or Mothership, and not armored units, so you're much better off going heavy Mutalisk against Viking harassment vs. Terran.", or "When upgrades Roaches and upgraded Marines engage, the regenerative properties of a burrowed Roach more or less nullifies the stim DPS bonus; so it's completely viable to wait until a stim, burrow and retreat, and then come back when the stim ends rather than just retreating above ground. You take less damage even though you move slower, and it leads to the T way overstiming." Husky and HD have a 0 in this. Artosis fails on this hard, Psy isn't great on it, and a few others need to learn a lot more.
C) Be entertaining, quick witted, and insightful. Tasteless isn't very good at this, and a few of the Youtube casters are awful on this.
D) Be aware of the goddamn action. I don't care how good of a caster you are... If the camera isn't on an engagement, you've failed. (There's someone who's really bad for this, but I forget who.)
In conclusion, Day[9] is the best caster by far, Tasteless is the second best, Idra is up there, and if he actually started doing it, InControl would beat them all.
None.