Bacteria is the most likely thing we would find, though I can't rule out fossilized bacteria on Mars.
And while finding single celled life elsewhere in our solar system increases the odds life is out in the universe, I think the "Great Filter" that life faces to evolve was going from single cell to multi-cell. It had to shift from simple no-nucleus or supporting mitrochondria prokaryotic cells to more complex Eukaryotic cells, and only those could become multi-celled. And for a billion and a half years, they didn't. So for most of Earth's history, the world had life but nothing complex.
Finding simple life elsewhere means the odds of life are better. If we find COMPLEX life, the odds of other intelligences skyrockets, because it means multi-celled life from which it arises is more common.