Question

What's the highest base power move that an unevolved Pokemon can learn via level up the earliest?

Comments

Can you clarify what you mean by the earliest? If a Pokémon learns a 50bp move at Lv 5 but another one learns a 30bp move at Lv 3, which one fits your question better?

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The Regis learn Explosion at Level 1.

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@Hexahedron The former.
@Amethyst single-stage Pokemon doesn't count.

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A lot of Pokemon can get Explosion at Level 1 from breeding or TM. Does that count?

Answer 1

This answer is only counting the level-up learnsets of unevolved, multi-stage Pokémon, and since it wasn't specified in the question, this will be for Gen 9.

Seedot learns Explosion (250 BP) at Lv. 33. This is the move with the highest BP and Seedot learns it at the earliest level out of all Pokémon who learn it through level up.

Chingling learns Last Resort (140 BP) at Lv. 16.

Toxel learns Belch (120 BP) at Lv. 1. This is the strongest move learned by a Lv. 1 Pokémon.

There are also lots of base stage Pokémon who can learn 150 BP moves like Hyper Beam and Giga Impact, but none of them get them any earlier than Seedot gets Explosion. The lowest is Makuhita, who learns Focus Punch at Lv. 34.

Source: Experience and PokémonDB Attackdex