The 2017 Bolts were produced between late 2016 and mid-2017. Subsequent model years follow the same pattern and there were no 2016 model year Bolts.
The 2017 to 2019 model years have the identical battery capacity and drivable range. Functionally speaking, 2017 and 2018 are nearly identical. 2019 saw minor changes such as slightly improved front seats and more control granularity (max battery charge level can be set between 40 and 100% range, and the heat & A/C buttons got separated).
2020 is set to have 10% more battery (60 -> 66kWh) and as a result, 9% more range (238 -> 259 miles). Charging profile has been improved so that, while the maximum charging speed remains the same, the weather or remaining charge has less effect on slowing down and so the resulting charging time is reduced. Other improvements include the installed cameras getting upgraded to HD resolution.
If you don't have a burning desire to get a 2020 Bolt, then any model years between 2017 and 2019 are going to be just fine.