Wow, what a question. There are so many options for under 500 bucks. I love my WSM, but for me to say it is the best for under 500 bucks, would be like saying what the best car is for under 30,000 bucks.
Without a doubt, a WSM is in the running. I love mine. But I've heard that a UDS is the best smoker on the planet, if you have the skills to build one.
Bottom line, a good BBQ cook can cook good BBQ on a used Weber Kettle bought on Craigslist for 20 bucks. Good technique is more important than equipment.
When I get too good for my cookers, I'll replace them. I'm not there, yet.