They may actually already have that dubious distinction. Right?
I had to look this one up and I was surprised at how low my estimate was... The thinking is that there are approximately one quadrillion (1,000,000,000,000,000) ants living on the earth at any given time! LOL After a billion (give or take), who's counting?
Tardigrades? They have been proven to survive anything, including dessication, the hard vacuum of outer space, primary cosmic radiation, nuclear irradiation and temperatures ranging from 400 degrees to minus 150. They're almost as hard to kill as Keith Richards.
Either with the extinction of humans, or further down the line, probably Nematodes will be the last standing (or crawling or drifting) animal on Earth.