If two layers conflict in determining a creature's base power and toughness, which layer wins?

Layers don't truly 'conflict' — the Comprehensive Rules establish a rigid sequence (Layers 1–7) in which continuous effects are applied, and each layer builds on the previous one. Base power and toughness specifically is handled in Layer 7a (CR 613.3a), which sets a creature's base power/toughness via characteristic-defining abilities (CDAs) or effects like Godhead of Awe.

Within Layer 7a, if two effects both try to set a creature's base power/toughness, they don't cancel each other — they apply in timestamp order (CR 613.7), meaning the most recently created effect applies last and thus 'wins.' If one effect depends on another, dependency rules (CR 613.8) may override timestamp order to apply the depended-upon effect first.

Sublayers 7b (power/toughness modifications like +1/+1 effects), 7c (counters), and 7d (power/toughness switching) always apply after 7a, so effects in those sublayers never conflict with base-setting effects in 7a — they simply stack on top in sequence.

Concrete example: A 2/2 creature is affected by two spells: one sets it to 3/3 (base, timestamp 1) and another sets it to 5/5 (base, timestamp 2). Both apply in Layer 7a. The 5/5 effect has the later timestamp, so the creature ends up as a 5/5. Any subsequent +1/+1 effect then applies in Layer 7b, making it 6/6.

In short: the 'later timestamp' effect in the same sublayer is the one that sticks, per CR 613.7b. True layer conflicts don't exist — the system is sequential by design.