Best sort of sales job for experience?

So recently I have been looking at changing jobs. I wanted to get a sales job because I know it's an important skill to have and is a really productive use of my time. Though I am having trouble figuring out what will be the best in terms of:

* Cold calling jobs
* Retail stores
* Internal sales

Was thinking cold calling. I am a bit scared of it and worried I was I'm going to quit fast but I do see good potential to it.

I looked at retail stores and such but if I am selling things like...

Best sort of sales job for experience?

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