It's a bit tricky to say definitively which constellation is the "closest" to Leo since constellations are just patterns we see in the sky, not physical groupings. However, if we consider the constellations that share a border with Leo, the closest would be:

* Cancer (the Crab): Located to the west of Leo.

* Virgo (the Virgin): Located to the east of Leo.

* Ursa Major (the Great Bear): While not directly bordering, it's very close to the northern edge of Leo.

You could also argue that Hydra (the Water Snake) is close, as it stretches across the sky near Leo's southern border.

So, depending on what you consider "closest," you could say Cancer, Virgo, Ursa Major, or Hydra are the closest constellations to Leo.