Chorus is a modulation based effect which relies on an LFO, or low frequency oscillator, to actively affect two major parameters of your audio track you apply it to, namely its timing and pitch. As I’ve covered in my many chorus tutorials, chorus can be used to add width to a track or tracks, achieve that trademark lush evolving tone, or create other interesting aesthetic effects. With that in mind, I thought I would offer up my pick for the best chorus plugin and offer a very competitive free option, as well, for achieving all of these effects in your mix.
The Best Chorus Plugin

Arturia’s Jun-6 Chorus plugin is my pick for the best chorus plugin for two reasons: its simplicity and its sound:

The plugin is named as such because it’s modeled after/to capture the warm stereo effect from the incredibly popular Juno-60 synthesizer from Roland in the 1980’s which was all over pop records of that decade.
In terms of its simplicity, you can see there are three buttons on the left: Mode 1, Mode 2, (plus a sum button to combine the two in mono) and Manual.
Modes 1 and 2 are EXCELLENT presets which I use more than half of the time for capturing in that classic, lush, evolving chorus tone, in particular Mode 1, with Mode 2 featuring a slightly increased Rate and Depth.
It really doesn’t get any easier; you simply drop an instance of this plugin on an Aux/Return track set to Mode 1 and 100% mix, then blend to taste on any track in your mix via its respective send dial to achieve some warm analog width at low levels, or more of that evolving sound at higher send ratios.
The “Manual” button is there if you want to set your own custom Rate, Depth, and Phase. Just a quick reminder:
Rate
The chorus “Rate” parameter is measured in Hz and determines how quickly the transitions set by the “Depth” parameter occur. You get the effect of a fast, choppy phaser when you turn this up. Turn it down and you get a much slower evolution, so slow to where you won’t hear it changing when set to the minimum. I find .4Hz is around the sweet spot… possibly because it sounds a lot like Mode 1.
Depth
The “Depth” parameter is measured in milliseconds and controls the range the rate covers. Keep this low to keep the effect more subtle or crank it up (I like it up around 2/3 to around 4-5ms) to give the best chorus plugin more ground to cover in that evolution of timing and pitch.
Phase
The “Phase” controls how dissimilar it is from the original/dry audio’s phase (see my overview on phase issues). Setting this to the max (180 degrees) creates the most width when combined with enough Depth.
Here are my favorite/the best chorus settings on the best chorus plugin:

The Best Free Chorus Plugin
While my go-to is Jun-6, a very serviceable and best of all FREE chorus plugin which I can recommend is OSL Chorus from Oblivion Sound Lab:

As you can see, it has additional parameters which Jun-6 doesn’t have. Aside from the Rate and Depth (the latter of which is measured as a percentage), you have Offset (which is somewhat another way of delivering the “Phase” parameter of Jun-6) and Filter to low pass and roll off the top end for a warmer or brighter sound.
Lastly, you note that it has separate Dry and Wet controls, offering up a different way to achieve the blend that you’re looking for with more precision.
If you’re using this as an Aux/Return track for sends, you’d again turn off the Dry and leave the Wet at 100% to ensure any amount you blend in via the send dial for that track exclusively is bringing the complete chorus effect to your already dry audio.
Download it for free and try it yourself (it’s both 32 bit and 64 bit compatible).
Let’s also give some love to the stock plugins out there. Ableton Live’s Chorus-Ensemble plugin has the “Classic” setting for creating the chorus effect on your track of choice, even featuring a width parameter for adjusting the size in the stereo field to taste:

As an aside, I did a complete overview on every one of Ableton’s stock plugins, so refer to that for more information.
Whichever chorus plugin you’re using, once again make sure you check out my many chorus tutorials for specific tips on how to use chorus in different instruments to get the most from this modulation based effect in your next mix.