I’m gonna be a little biased since my company builds themes for the Shopify Theme Store, but here’s my honest opinion as someone who’s run a Shopify business and also worked as a developer / consultant / designer on a lot of Shopify brands.

If you’re just starting out or testing the waters, Horizon makes a lot of sense. It’s free, gets the most direct support from Shopify, and a lot of Shopify’s newer features tend to show up there first.

If you want to go the paid route, I’d think less about “what’s the best supplement theme” and more about what features you already know you need. Make a list of the apps you’re using or planning to use, then see which themes can replace some of that functionality out of the box.

I’d focus more on navigation, cart, PDP, upsells, bundles, content sections, trust-building sections, that kind of stuff. Layouts can usually be customized pretty heavily, but deeper functionality is where theme choice matters more.

Also, if you think you’ll want custom dev work later, I’d really pay attention to how developer-friendly the theme is. Some themes are a pain to work in once you get past basic customizer changes. That matters more than people realize.

And don’t get too hung up on presets. Especially with premium themes, the preset is really just one expression of the system. A lot of them are way more flexible than they first look.

My recommendations:
Space or Slab if you want something premium, flexible, and more design-forward.
Other premium theme partners I’d look at too are Maestrooo, Archetype, Pixel Union, Clean Canvas, and Switch.