Generally speaking, look to applications that (x) sell to SMBs — especially very small businesses and individuals — and (y) are successful.
SaaS products purchased by individuals cannot incur any meaningful sales or marketing costs. The vast majority of their new leads and customers have to come from viral, brand, and other close-to-low-cost acquisition channels.
As a result, their products tend to have the best user experiences (if not design) because they cannot afford to invest in a human-intensive onboarding, sales, marketing, and deployment process.
Vendors selling to the enterprise, by contrast, can paper over rougher design (especially on UI) with the power of complex workflows, and also having humans help with onboarding and business process change.
Some to look at include Trello, Pipedrive, Automile, Mixmax and others that have grown very rapidly “selling” to very small businesses and individuals.
The line between a consumer product and an SMB product can be fairly fine here.
Similarly, look to successful, fast-growing products with very effective “long tails”, i.e. with 1000s of customers on cheap and free programs. These also similarly tend to almost have to be well designed to scale. Examples include Twilio, Algolia, Sendgrid, Stripe, etc. on developer side and many others, as well as obviously candidates like Slack and DropBox.
You may take issue with some of these examples, but if you look at all of them as a cohort, you’ll see a set of very successful apps in different genres that have all scaled with at least an initial CAC close $0 … that equals well-designed in my definition of what really matters.