Well, Its depend on How your server configured and wich script or plugins you use on your website.
For example, if you host static HTML page, $5 VPS will be enough for thousands of visitors.
I suggest you, start with $5 plan if your site is fully new. $10 if you have good traffic. You can upgrade to upper plan anytime using digitalocean control panel. Also, use Nginx instead of Apache.
In my opinion, DigitalOcean + ServerPilot + CloudFlare works best.
also install W3 total cache if its wordpress site.