1. Wrong: Of my sister and me, I stand the tallest.
Better: Of my sister and me, I stand taller.
Also better: I stand taller than my sister.
Explanation: With only two people's heights being compared, the comparative rather than the superlative makes sense.
2. Wrong: Sasha has the most hardest last name to pronounce.
Better: Sasha has the hardest last name to pronounce.
Explanation: Hardes is already the superlative form, so a second intensifier isn't necessary.
3. Wrong: Terry and Val, their classmates all agree, have the most best enunciation on the Debate Team.
Better: Terry and Val, their classmates all agree, have the best enunciation on the debate team.
Explanation: Most and best serve the same function, so both aren't necessary.