When all the animals on the planet and fish in the ocean are extinct, the ants will still be here.
It will be the ants vs. the cockroaches one step before that. Yeah, my money is on the ants as well.
Anyway, the entry on ants at Wikipedia contains a link to the Schmidt Sting Pain Index that is too good not to share (useful reference):
It’s been shown that scorpions are very sturdy and will most likely be survivors long after man is gone as well.
They have found ants, roaches, rats and scorpions near nuclear test sites after the blasts and they are all able to take the heat and radiation of the bomb without much effect.
I’ve seen scorpions frozen in a block of ice and thawed out with a blowtorch and they just walk away. Also, a scorpion can go for 1 year without eating or drinking.
Insects are very strong and will outlast us for sure.