Bacteria would be something that would be really cool to investigate. For example, if you took a colony of a million of Deinococcus radiodurans. Put them in conditions where the temperature is -79 degrees and zap them with gamma rays equivalent to those on Mars, the average population size would only reduce by one bacteria every year.
I think we’ll end up finding bacterial life of some sort on other planets. Bacteria are just too capable of surviving in crazy environs that we never expect.