Dismalhead
Poster Extraordinaire
I'd guess a Fender Frontman that comes with the Squier Affinity Strat kit. So many on CL.

Define,"best".
Quantity? Profit margin? Speed at which inventories are depleted?
Best is subjective, but simply based on longevity, the Peavey Bandit has earned a place in the pantheon of guitar amp history.
Ok, quantity.Best-hyphen-selling, means "the one that has sold the most units". Having nothing to do with "best quality" it is not subjective... just hard to find data.
I have heard HRDX, but recently I've heard Mustang, No clue if that's actually true
Judging from what I see on Craigslist listings, it appears to be Line 6 Spiders.
I'd like to correct myself...Fender claims that the HRDx is the best selling TUBE amp of all time.
Demand declines with price. The HRDX is an expensive amp, if you consider all amps. No way that's it.
I'm still thinking something under $100: Peavey Backstage, Fender Frontman, or the like. Hell, it might even be a Gorilla.
<ducks>
Maybe Fender sold more Mustang amps that any other model in 2013, say, but I find it hard to believe it's racked up so many units that it is the best selling amp "of all time".