If I'm to consider strength & weaknesses here, then my vote goes to Octavia. Her instrument can be used as a baseball bat against a flying ball of fire heading straight toward her. Or just point the end of the instrument at Spitfire and stab away. It'll be short in the pegasus category for round 3 and wanted to vote for Spitfire, but Octavia has a better shot at winning Round 3 in my book against upcoming opponents.
This is for everypony. You have to think about their skills and abilities, don't just pick your favorite pony or just picking at random. Think of their strengths and weaknesses and their position in the MLP universe.