We're big fans of the Gliese 581 system here at DVICE. It's been a treasure-trove of potentially habitable exoplanets, and the latest data boosts one of them, Gliese 581g, up to a whopping 0.92 on the Earth Similarity Index. Yes, we now have one of those.
The Earth Similarity Index (or ESI for those in the know) is a mathematical formula that relates a number of planetary properties to their corresponding values here on Earth and then spits out a score where zero is as far from Earth as you can possibly get, and one is nearly identical to our home planet. Here's what the numbers mean:
As a general rule, any planetary body with an ESI value over 0.8 can be considered an Earth-like planet. This means that the planet is rocky in composition (silicates) and has an atmosphere suitable for most terrestrial vegetation including complex life. Planets with ESI values in the 0.6 to 0.8 range (i.e. Mars) might still be habitable too, but only by simple extremophilic life, as they are either too cold or too hot, assuming life as we know it.
And here's a nifty graph showing how 258 exoplanets (and all the other stuff in our solar system) stacks up against Earth:
That blue square in the upper right corner is Gliese 581g, and as you can see, it's by far the most habitable exoplanet that we've ever found, at least on the ESI. It's not necessarily habitable for us — at up to twice the size of Earth, spending time there might not be particularly enjoyable for humans — but it's certainly possible that a bunch of other stuff is already living there anyway, since the planet is solidly in that "warm and wet" Goldilocks zone. Plus, the Gliese 581 system is only 20 light years away (a mere asteroid's throw by galactic standards), making it an appealing place to head for as soon as we get warp drive working just 50 years from now.