How can I show just the most recent post on my home page with jekyll?

<ul class="entries">
  {% for post in paginator.posts %}
  <li>
    <a href="{{ post.url }}">
    <h3>{{ post.title }}</h3>
    <p class="blogdate">{{ post.date | date: "%d %B %Y" }}</p>
    <div>{{ post.content |truncatehtml | truncatewords: 60 }}</div>
    </a>
  </li>
  {% endfor %}
</ul>

That shows all my posts, I just want to show the most recent.

This can be accomplished through the use of limit:

{% for post in site.posts limit:1 %}
... Show the post ...
{% endfor %}

You can also use limit and offset together to "feature" your most recent post:

<h1>Latest Post</h1>
{% for post in site.posts limit:1 %}
... Show the first post all big ...
{% endfor %}
<h1>Recent Posts</h1>
{% for post in site.posts offset:1 limit:2 %}
... Show the next two posts ...
{% endfor %}

Rather than create a loop, just assign the variable and move on...

{% assign post = site.posts.first %}

(Edit 2018) Since someone wanted to know how to iterate other posts after you've done this:

{% for post in site.posts offset:1 %}
  ... Show the next posts ...
{% endfor %}

If you got here for the question as stated in the title, "How can I show just the most recent post on my home page with jekyll?" and not "how do I show only the latest post in my template," the following might be helpful.

Given a brand new Jekyll version 3.7.3 install with the default minima theme, create a file, _layouts/home.html with the following content:

---
layout: none
---
{{ site.posts.first }}

Causes Jekyll 3.7.3 to show the first post, using the post template, as the home page.

It appears you can also just access the latest post via the first index of site.posts as in:

{%- assign latest_post = site.posts[0] -%}

Latest post: <a href="{{ latest_post.url }}">{{ latest_post.title }}</a>

While site.posts.first works too as mentioned by someone else, the above example also provides a consistent manner for accessing other indices besides just the first (not that you would ever need to). Also, I didn't have enough reputation to add this answer as a comment instead :)