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What have been the top findings on Mars so far?

Robert Walker, Approaching this as a mathematician, interested in the planets

Answered Jun 21, 2014

I'm especially interested in potential for past and present life so that informs this list, though also added a couple of things I just think are really fun and cool :).

Discovery of shorelines of ancient ocean - more and more evidence confirming it - including recently - study of deltas of rivers that used to flow into it.
Artist's impression of ancient ocean on Mars - this is actually a second ocean believed to have formed about a billion years after the first one - didn't last so long, first one lasted for hundreds of millions of years probably though it may have been covered at ice at times towards the end, best hypothesis is that it started off at hundreds of degrees C in dense CO2 atmosphere same as for Earth.
Discovery of the river channels, detlas and extensive flood features

Eberwalde Delta
Discovery of the Warm Seasonal Flows - and then, discovery of them in the equatorial regions of Mars last year - clearest indication of possibility of a habitat on present day Mars
Curiosity's discovery that the ancient deposits were far more habitable for life than expected, plus finding organics also - both suggesting that conditions on Early Mars were conducive for development of life there
see New Results Send Mars Rover on a Quest for Ancient Life
This goes back to Viking - rather surprising discovery - photos of white frosts on Mars even in the equatorial regions. This is because it gets so very cold and though there isn't much water, not much absolute humidity - night time relative humidity is 100%. And recently DLR found that a few hardly Earth lifeforms can photosynthesize and metabolize using just this humidity without any water.
Phoenix isotope measurements showing that Mars has been geologically active in the recent past exchanging CO2 with the atmosphere - and that the O2 in the CO2 has subsequently exchanged atoms chemically with something else - almost certainly water - so reasonably clear evidence of surface water there somewhere either episodic or all the time in recent geological past
Also visual confirmation that some of the volcanic craters show signs of reasonably recent eruptions

Here the blue regions of the Olympus Mons caldera are thought to be around 140 million years old - relatively recent. Other evidence also of geologically recent activity of Olympus Mons. See Timeline of Martian Volcanism and Lava flows at foot of Olympus Mons. No known present day geological hot spots or activity anywhere on Mars, but may get lucky and spot something. This is also relevant for search of life as it suggests possibility of geological hot spots - some possibly close to surface - which could melt ice and create habitats for life.
Phoenix discovery of salts on Mars in bed of ancient ocean - and surprising result that they are constituted of sulfates and perchlorates rather than the expected chlorides we get on Earth
Deliquescing salt drops on Phoenix legs - at least most think that's what they are - showing a way that you could have habitats for life on Mars simply by salts absorbing moisture from the night time humidity (a method that microbes use to survive on Earth in the extremely arid heart of the Atacama desert)
Drip-drop Mars-style
Discovery of hydrothermal vents on ancient Mars - some of them exposed to view so we can explore them with future missions

Volcanic cone in Nili Patera Caldera. Arrows point to light coloured hydrothermal vent deposits, which show that it must originally have been warm and wet (in first few hundred million years of Mars)
Discovery of deep deposits of clays on Mars - this makes it possible to study ancient Mars as a stratographic sequence when we get the opportunity to visit them with our rovers
Close up image of a region of stratified clays in the Mawrth Vallis region of Mars
Discovery of the Martian Geysers - these are just really cool things, dry ice geysers - but the method that forms them - solid state greenhouse effect - could also melt water beneath ice or dry ice and create life habitats as well.
Artist's impression of what they might be like (enhanced colour of course) NASA Findings Suggest Jets Bursting From Martian Ice Cap - NASA Jet Propulsion Laboratory
The grooves left by sliding blocks of dry ice on Mars - these - not particularly to do with search for life but is really fun cool thing you don't get on Earth.
Dry Ice "Snowboards" on Mars
Another fun cool thing - discoveries of meteorites on Mars. Because of the thin atmosphere, they can survive to the surface more easily and Opportunity for instance has found several. It's also relevant to search for life.
Mars Exploration Rover Mission: Press Release
Larger meteorites impacting on the Martian poles could create temporary habitats - ice covered lakes that could last up to a thousand years.

It's relevant to search for life because of the possibility of meteorites on Mars from Earth - and question - does this ever transfer life from Earth to Mars - perhaps every few ten million years - only extremophiles able to withstand cosmic radiation and vacuum of course. Nobody knows. So far, no Earth meteorites found on Mars.

Larger meteorites could create habitats on Mars for Earth life - a big meteorite colliding with the polar icecaps could create an ice covered lake that stays liquid for a thousand years. Again no evidence of these yet.
Again just rather cool - the videos of the moons eclipsing each other from the surface.

More fancy Phobos and Deimos photography by Curiosity
Curiosity video of eclipse of sun by Phobos
Blue sunsets