Required tags on every public page

<title>: unique per page, 50–60 characters displayed (longer values just get truncated with an ellipsis in search results, so keep the important words first).
<meta name="description">: unique per page, ~150–160 characters, written as genuinely compelling ad copy for that specific page — not keyword-stuffed, not copy-pasted across pages.
<link rel="canonical">: the single preferred URL for this content, absolute (full scheme + host), even if the current URL is already canonical.

og:title, og:description, og:image, og:url, og:type on every page that might be shared — without these, link previews on social platforms and chat apps fall back to guessing from page content, often badly.
og:image must be an absolute URL, at least 1200×630px for reliable rendering across platforms, and should be a dedicated image per page (or per page type) — a single shared site-wide image makes every shared link look identical and less clickable.
Set og:type accurately (article, website, product, etc.) — it changes how some platforms render the preview card.

Twitter/X card tags

twitter:card (usually summary_large_image), twitter:title, twitter:description, twitter:image — these can duplicate the Open Graph values, but include them explicitly rather than assuming a platform falls back to og:* tags correctly in every case.

For any templated page (product pages, listing pages, user profiles), generate meta tags from the actual entity data, never a single static fallback — a fallback title/description repeated across thousands of pages is a duplicate-content problem, not just a missed opportunity.
Regenerate og:image per entity where feasible (a dynamic OG image with the entity's title/key data) rather than one static image for an entire content type — it measurably improves click-through from social shares.

Never leave placeholder/example meta content ("Lorem ipsum," a template's default title) live in production — audit generated pages after any template change, since a bug can silently apply a fallback to real pages.
Don't forget noindex directives on pages that shouldn't be indexed (internal search results, filtered/paginated duplicates beyond the canonical page, admin routes) — meta tags and indexability are two different concerns and both need explicit handling.