Because the other answer is bad, ima go with the answer of Ice Beam. Fire Blast, unlike the other answer says states, is probably the worst choice here.
Ice Beam covers Grass, Ground, Flying and Dragon.
Fire Blast covers Grass, Ice and Bug
Thunderbolt covers water and flying.
Already we can see Ice Beam covers the Grass in Fire blast and Flying in Thunderbolt, and as we see far more Ground and flying types than Ice and Bug types (that threaten Mewtwo), Fire Blast is a bad choice here. Thunderbolt covers water, but that’s about it. Ice Beam is the best one for Mewtwo.