Honestly, the best strategy game ever.
Firstly, scale. There are a huge range of maps available to play on, with vast oceans, mountainous plateaus, and broad plains, allowing key interplay of land, naval and air units. The maps themselves range from massive (taking the fastest units several minutes to traverse their length, and the slower ones anything up to half an hour) to tiny (tens of times smaller). Even the units themselves follow this trend, with the largest capable of destroying literally thousands of the smallest, and also costing thousands of times more than them. This is added to by the use of nuclear missiles and artillery that can shoot across entire maps.
The other amazing thing about Supreme Commander is how that for every challenge, there are thousands of ways to solve it. This variability manifests itself everywhere, throughout the game, and stops the problem many games have of two good players ending up in a monotonous stalemate. There are hundreds of available units with as many specific uses, including many that would be superfluous in a lesser game. For example, aircraft are not simple hovering units which can be sent to attack enemy forces wherever and whenever you feel like it: real life strategy is simulated with aircraft being very weak to anti-air which can attack them back. Their mobility means they can be used to launch attacks on undefended enemy positions and units quickly, and can refuel (as aircraft can only be airborne, as in reality, for a certain time, adding to the strategy) and be repaired quickly at particular units (e.g. aircraft carriers) and buildings. Instead of hovering, the planes whizz rapidly and realistically around the sky, banking through the air and dropping bombs, while the slower helicopter gunships move more slowly, sacrificing speed for durability and firepower. This is simply an extended example to help understanding of the layered strategic scenarios in Supreme Commander, and land, naval, experimental (i.e. absolutely gargantuan units), artillery, and missile systems all have a rich strategic network. You can play the campaign, multi-player and versus AI modes for many, many hours without repeating yourself in strategy.
As for the rest of the game play, suffice to say that there is a deep, many sided storyline and campaign, three different factions which, despite having only slightly different unit combinations, lend themselves better to radically different strategies. The graphics are awesome; just check out some of the pictures with the game, and are completely different for each of the factions. It is really quite incredible watching you enemy endeavour to destroy your immense experimental units as they converge on their base!
Overall, this is simply a game which in so many different ways completely flattens almost every single strategy game before and since, despite being a few years old.