It's a tie between summer and fall for me.
As several other posters have stated in this thread before me, summer in the PNW is very good.
But there's something about fall -- maybe it's the red-and-yellow color scheme, the leaves falling, the weird but cozy-feeling dark blue sky that appears for about a week and doesn't come back until next year -- that just gets me. It's not as cold or depressing as spring/winter, but it's not nearly as hot as summer.
Despite me staying inside 90% of the time to use the computer like a weirdo, I think I have some form of seasonal affectiveness disorder, so even just seeing the blue sky outside makes me feel a teeny bit happier. A teeny bit. So that's why I like summer.
Winter is only good if it snows. Which is a bit of a crapshoot up here. (Fuck rain, though. I don't get why everyone else likes it so much. I don't like getting my hair wet... (although, I will admit, hearing the rain from inside is nice.))
Spring is eh. I don't hate it, but over here it just feels more like Winter Lite, so.
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I don't know what to put here.