What is THE most important thing to learn for bass? It's a simple question, but maybe not so simple of an answer. I figure everyone will have a different answer, so this should be fun.

You can answer anything specific, but nothing so broad as a genre or style. It can be a scale, technique, song, etc. And if you'd like to answer different things for different genres, feel free.

BTW, I know that there is no "most important thing", but this is a game so play along.
...it was bright as the sun, but with ten times the heat
i'd agree with Hail - it's listening. most bass players i know would say timing, and that's important, too, but if you really listen - to your playing, to the music you want to play, to other players, you can tell what you do and don't like, what does and doesn't work, and what you love to hear.
Quote by Skeet UK
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Get a drum machine,or a metronome and play with it all the time. It will help you by training your ear to lock and listen to your fellow rhythm section members and solidify your groove. A band with a solid rhythm section can make or break a band. It will also start build those skills of playing to the song and serving it well in the lower end
I wouldn't say it's the most important thing (sorry to break your thread), but I'd say it's the most important thing that's very often overlooked... some basic knowledge of the physics of sound, how the instrument works, how amps/speakers work, how to use an equaliser etc. I see too many people with decent chops and awful tone.
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Rhythm, my friend. A bassist without rhythm - and a good sense of timing - is like a swimmer who cannot swim.
"Maybe this world is another planet's hell?" - Aldous Huxley
Listening is something you need as musician rather than specifically as a bassist: any decent musician needs to be able to listen carefully and pick out the elements that may be improved in their playing.

Rhythm is, too, important for all musicians, but even more so for a bass player, as the bass usually plays a rhythmic role in the song. I agree, playing in rhythm, and knowing your way around rhythmical figures and the such is perhaps the most important thing to know as a bass player.
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Quote by Hail

^^THIS^^ Particularly for Bass, its Drums. Then listen to how it fits in with the vocal (if any) and other instruments.

Learning music is all about listening. Different instruments listen to different things.
Find an instrument that fits your left hand and inspires you to play things you did not think you could.
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