If you want something barebones with the smallest possible transaction fees and a good API, Helcim was quite good at least a few years ago when I used it. This was for a project that did a lot of micro-charges and they were the only payment processor I could find where that would still be profitable because of their (then) low fees. Not sure what their fee structure is now, though.

Adyen seems to be a fair bit cheaper, no idea about if they are good or not

All the big boys run on Adyen.

Adyen seems to be missing a huge chunk of Stripe's functionality: subscriptions, billing portal, etc.

Do you have any examples or could you mention some industries?

I worked briefly in sales at stripe, and most large enterprises are on or have considered Adyen. Keep in mind that many of these businesses will use more than one payment solution.

The business I’m working at now splits traffic between adyen, stripe, and others. Anecdotally, stripe has the best payment success rate of any of our providers.

Would love to learn more about that business -- I've pondered trying to build a payments API that could offer a way to transparently switch between payment providers, hadn't seen too many companies that were doing it

I know of Chargebee, Recurly, etc but I'm not sure those companies will let you actually spread payments between or if you pick one provider and use that. I've had people ask me if I knew of a company/product that could let them be less reliant on Stripe alone.

Vueling, Booking.com, Dropbox for example.

Interesting that Dropbox don't use Stripe? Seems a bit surprising as I thought YC companies generally use each others wares... I think the club aspect of YC is one of the most powerful to be honest.

In my most recent invoice view, this is in their JS code:

"adyen-checkout": "prod_assets_web_modules/adyen-checkout"

Adyen and bolt seem to be popular.