head to head
Neither is simply harder - they are hard in different places. Grand Theft Auto V demands more reads; Mega Man Zero demands more execution.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.

Execution · Practiced Mastery
Zero awakens to lead a resistance in a harsh, rank-graded GBA action series debut.
Grand Theft Auto V leans Execution (Practiced Mastery); Mega Man Zero leans Execution (Practiced Mastery).
Mega Man Zero leans clearly harder on execution.
Grand Theft Auto V
Driving and shooting demand decent inputs.
Mega Man Zero
Punishing GBA action with strict ranking on damage
Grand Theft Auto V leans clearly harder on reads.
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Mega Man Zero
All bosses are pattern-based
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Grand Theft Auto V
Some mission and lobby randomness.
Mega Man Zero
Cyber-elf locations hidden at first
Mega Man Zero leans clearly harder on optimization.
Grand Theft Auto V
Heists, money loops, builds learnable.
Mega Man Zero
Rank system penalises elf use, rewarding optimised clean runs
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.