A good office chair (not necessarily one of those expensive as fuck mesh ones - I hate those… But something quality).
Man, I get they're not for everyone, but after having a mesh chair, I will never go back. Currently on my second one in about 8 years, so it's not exactly BIFL material but the first one lasted longer than a 'normal' chair ever did, and neither were particularly expensive, as quality chairs go (I paid ~$150 for the first and ~$225 for the second, got both during sales, so I'm not sure what the regular price would have been but I'd guess $300 or so).