head to head
Neither is simply harder - they are hard in different places. Donkey Kong 64 demands more optimization; Grand Theft Auto V demands more reads.

Optimization · Practiced Mastery
The N64 collectathon that buries five playable Kongs in collectibles.

Execution · Practiced Mastery
Sandbox crime playground spanning shooting, driving, and a persistent online economy.
Donkey Kong 64 leans Optimization (Practiced Mastery); Grand Theft Auto V leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Donkey Kong 64
Moderate platforming and minigame dexterity
Grand Theft Auto V
Driving and shooting demand decent inputs.
Grand Theft Auto V leans clearly harder on reads.
Donkey Kong 64
Scripted bosses and enemies
Grand Theft Auto V
Online PvP exists but mostly gear/positioning, not modeling.
Grand Theft Auto V leans a little harder on variance, though the gap is small.
Donkey Kong 64
Little randomness; secrets known once found
Grand Theft Auto V
Some mission and lobby randomness.
Donkey Kong 64 leans clearly harder on optimization.
Donkey Kong 64
Routing five Kongs through collectible checklists
Grand Theft Auto V
Heists, money loops, builds learnable.
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.