Straightforward answer and also quite controversial is NONE.
IMO: A plugin alone won't optimize Core Web Vitals overnight. Core Web Vitals at the moment are LCP, INP, and CLS. Let apart INP, which around 90% of WordPress websites pass easily. With a plugin either a caching (such as WP Rocket) and/or an optimization (such as Perfmatters) one, you could "optimize" (better to say, improve) only the LCP metric. The LCP metric is influenced by (or even better, made of) FCP and TTFB, the first one can be optimized using an optimization plugin, by removing unnecessary resources and prioritizing essential ones; while the second one can be improved by setting up proper caching or even better using a CDN. Then it comes the LCP itself, which is usually an image, but it can also be a video or text, and here comes into play Perfmatters (or FlyingPress, which is another powerful plugin) which can automatically prioritize and preload LCP content based on page/post (with the required tweaks into configuration).
CLS, however, is another story, if your website fails to pass CLS and therefore happens to have a CLS issue you have to do proper debug and check for overlapping and delayed CSS rules and/or dynamic content inserted via delayed/deferred JS. But here comes a catch, by using an optimization plugin you could even create CLS issues yourself...since these issues typically occur when content on the webpage is being added/styled after initial loading or more in general delayed. And that's basically one of the main features of such plugins, delaying or deprioritizing the loading of resources.
With that being said, other members already pointed out Perfmatters which is indeed a very powerful optimization plugin. I almost always pair it with a caching plugin but sometimes I could end up having only Perfmatters with a CDN configured.
As per caching plugin, it depends on your web server configuration:
If it's a LiteSpeed or OpenLiteSpeed, go with LSCache (fully free, and really a beast).
Anything else than LiteSpeed, go with FlyingPress (more friendly and powerful than WP Rocket).
The last thing I would point out, some say maybe something like that CWV don't bring anything good or are a waste of time. Well, let me tell you this, obsessing with speed is what can make your website look different from the others and bring you up in search rankings. When you look at speed you focus only on web vitals (which are still a ranking factor) but in reality, the loading speed of a website can influence a bunch of other essential ranking factors and/or metrics such as bounce rate, time on site, page views, and conversions. A fast-loading website creates a positive first impression and improves user satisfaction and retention. Users are more likely to stay on a website and interact with its content when it loads quickly, resulting in a better overall user experience. So, I strongly disagree with that, it might be a waste of time for you but while websites are being populated with the same AI-generated content, Google has to find other ways to rank a website...speed might be the one.
