Most of the speculation about extraterrestrial life in the Solar System seems to be centred on Mars and Europa, but I think Venus might be a better bet.
I know, the conditions on Venus today are hardly conducive to life as we know it. But there must have been a time in the early history of the Solar System when the planet would have been very much like the Earth. It's about the same size; it must have been cooler before it suffered a runaway greenhouse effect (if that's why it's so hot today); and the Sun wasn't always as hot as it is now.
So life could have evolved. And if there's one thing we know about life - especially simple microbial life - it's that it's very hard to eradicate it once it takes a hold. Here on Earth bacteria have evolved that can survive in extremely hostile conditions. Why not on Venus? Why could simple life forms not have continued to evolve as the planet's environment deteriorated, adapting themselves to ever harsher conditions?
I'm going to stick my next out and say that some form of microbial life is flourishing on Venus to this day.