I apologise I'm going straight to Least helpful, because I've been trying to perfect scores* so these are fresh in my mind.
LCP. Oh boy, it's hard to track down what will make a difference. Is it the font, the CSS sizing of the element, the images in it... help on diagnosing what's causing the LCP time would be great.
But by far the least helpful metric is when I'm told that analytics.js could be optimised further. A (I assume) Google-sponsored tool is telling me I should improve the result from another Google tool. I appreciate why it happens, but it's frustrating :)
* Only to learn earlier this week from Paul Irish that some sites can't reach 100 due to the way the scoring curves are set.
I think the other thing is I find it hard to remember what the core web vitals mean, and to relate them to what's going on.
When we used to measure TTFB, DomContentLoaded and page fully loaded, I understood what those meant and could relate them to what's going on on-screen.
The core web vitals are a bit abstract to reason about - and how they're linked. FCP vs LCP, how they interplay...
I'd like a video where a page loads and PING! a vital happens, the page pauses and someone talks about that vital, what's led up to it, what's affected it and what would change it. Then the page unpauses until the next vital is reached. Something to make it relatable.
Suggestions/feedback on the Lighthouse guides is also welcome (the documentation that you see after clicking those "Learn more" links).
Disclosure: I work on Lighthouse and web.dev