In the Revolutionary War, there were a handful of Pennsylvania rifles (jaeger's) that caused a lot of damage in the hands of Morgan.
In the Civil War, the Whitworth's in the hands of CSA riflemen were pretty devastating. So were some of the rifles fielded by Berdan. Largely, I think, C. Sharps.
In WW1, the Ross rifles were pretty highly in-demand (created by my crazy great-uncle Ross... yes, there is a lineage there.)
In WW2, Springfields, Mauser's and Arisaka's...
So is there a most 'badassed? Well, IMHO, that's a pretty stupid term. Sort of Grand Theft Auto comes to history.
But are there some rifles that have played amazing roles in history? Certainly.
Suggest reading my friend Martin Pegler's book "Out of Nowhere -- a History of Military Sniping." That is about the best all-around look at rifles, tactics, etc. of generations of combatants.
I don't believe he uses the term "Badass" anywhere. But can let the reader judge the combination of technology and how well the tactics and technology matched up. From there, you can decide what platforms had the most impact for their time and technology.
It's a badassed book.
Cheers,
Sirhr
PS. I got my badassed and my baddest confused. Pardon my bad ass for not having the most baddest bad-assed post around.