What is the best nature for Bulbasaur?

Modest with grass and status moves.

+SpA -Atc

In-game anything goes, especially with Bulbasaur since it's well-rounded

But lowering attack is pretty much the best you can get

You don't need to worry about natures for in-game playthroughs. The only time you need natures would be the Battle Tower (but the RNG will fuck you over anyway so there's no point in even trying).

Bold, Modest, or Calm, depending on what you're going for.

Unless you intend to run a mixed set, which isn't that uncommon. Really, it depends on the set.

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Thank you