Well, I am pretty impatient, so I got my wife to mash down B with Sonic for an hour while I ran tests.
Findings:
Jab: always clanks; disadvantage for us.
Ftilt: sometimes clanks, sometimes wins.
Dtilt: always wins, but punished if we whiff.
Utilt: didn't try.
Usmash: always clanks; hard to time.
Dsmash: always clanks.
Fsmash: wins, but hard to time.
Throw: always trades (both take 3%).
Pivot Grab: almost always trades; won once in 40 tries.
Klaw: won once, couldn't reproduce. Usually trades.
Bowser Bomb: always wins.
Fortress: always clanks.
Nair: doesn't come out in time on reaction usually, but traded for 5% each when it did.
If we are going for the safest option, Fortress and Jab are the best bets. If you are confident, dtilt or Bomb beats Spindash decisively. Pivot Grab is not worth going for. Ftilt is a nice medium between the safe and risky bets.
Spot dodge was also solid, as Fortress beats Spindash after it turns around for a second hit. Most players jump and up B, though, so it might not matter.