The best vocal compressor varies from producer to producer. Every compressor has different characteristics, behaviour and sound. In addition, producers might use different compressors or more than one compressor at the same time in order to achieve the results they want.
The Best Vocal Compressor in My Opinion
I cannot prove which one is the best but in my taste the best vocal compressor is Waves Renaissance Vox compressor. The reason why I like Rvox the most is that it is very easy to use and it sounds nice.
When I watch videos of popular mixers, I saw several times that they like Rvox as well.
My other favorite vocal plugin is again from Waves and it is CLA-2A compressor/limiter. CLA-2A is the digital version of one of the most popular compressors of all time which is Teletronix LA-2A. It is also very easy to use and nice sounding compressor as well.
You can check the web pages about these products and read comments. Also you can watch videos of these products and decide yourself.