For my personal hobby projects I use Vultr. I used to use DigitalOcean and others but found that Vultr has better performance/price, both in my ad-hoc experience and in benchmarks when I was choosing. My needs are minimal, but I do like the private networking and IPv6, I've used separate storage volumes and metadata service, but not very much.
I agree with others that it's not a good idea for an actual company building a real product. You'll likely soon get into a situation where a using an available cloud service/feature will simply solve something avoiding a lot of infrastructure/development work for trade-off in paying the price for a service. Cloud-agnostic isn't a worthwhile goal for a 'true' startup, where calendar time and developer time should me things to optimize for more than cloud bills. The idea is to iterate/pivot quickly and using services to try stuff is the advantage. If this doesn't match your 'startup', that's fine and you can choose based on your criteria.
I agree with others that it's not a good idea for an actual company building a real product. You'll likely soon get into a situation where a using an available cloud service/feature will simply solve something avoiding a lot of infrastructure/development work for trade-off in paying the price for a service. Cloud-agnostic isn't a worthwhile goal for a 'true' startup, where calendar time and developer time should me things to optimize for more than cloud bills. The idea is to iterate/pivot quickly and using services to try stuff is the advantage. If this doesn't match your 'startup', that's fine and you can choose based on your criteria.