Which mammal has done the least amount of evolving?

Physical resemblance is meaningless. Many animals of differing evolutionary paths physically resemble each other because of adaptation.

Evolution is all about genes. So if you had said “What animal most closely resembles (genetically) its fossilized ancestors the farthest back in time?” I’d agree with you - and point out you’ve essentially asked the question I posted.

A bit of a hijack…my neighbor has two trees in his yard that are “living fossils”-they are the “Dawn Redwood” and the Ginko. Both of these species are very ancient…why have they survived into modern times?
I believe that the ginko was rescued from extinction by the Bhuddist monks at a monastery in China…otherwise thew species would be long gone by now!