Kind of surprised I did not mention this yet

Underrated (relative to popularity rather than how much people liked it)

Supreme Commander, the first one and it's expansion. An RTS from 2007 with Land, Air , and Naval forces. The relative strength of any given unit/structure is determined by which of the four Tiers it is in starting at T1 and progressing to T3 and the Experimental T4 units. The Three factions are similar in the first Tier, but the other technology branches offer unique units following the Doctrine of the specific faction. The resources are not fixed, rather fluctuating and generally increasing in rate over the match. Higher tiers grant more efficient resource gathering structures (generators for power and mass extractors for mass). Power is of little importance, as generators can be placed freely but there will be heavy contention in the mid game for the limited mass deposits. Time also becomes a minor resource as units and structures can be built or assisted by other engineering units in order to increase the speed of construction at the cost of draining resources faster. Why produce 3 light assault bots every minute when you can have one factory build twice as much in the same time? The only restricting factor is the rate you produce mass/energy and the time it takes for a unit to leave a factory upon being built. A fairly controversial tactic is to load up any transport aircraft with a couple of them light assault bots and harass the enemy engineers while trying to rush mass deposits with little protection. What's that? no, we are not going to attack from the ground like a fool. The LABs have the unique ability of having functional weapons even while being transported, you have just made a ghetto gunship. Next up, Naval units are particularly noted for having some of the strongest weapons outside of Tier 4 units, the crafty Cybrans noticed this and gave some of them legs effectively creating an amphibious fleet of death. The scale gets pretty epic too, almost cinematic when a few hundred units on both sides smash against each other till one is left standing. Lastly, in a fight between evenly matched opponents, where neither side is gaining much ground, there are the 'game ender' units. My favorite being the 'Yolona Oss' introduced in Forged Alliance. A T4 Experimental Missle Launcher that builds special missles so large, it requires two anti-nuclear missles to impact it before it is eliminated. Along with the fact that it builds not only Nukes on steroids, but in 1/8th of the time as a standard Strategic Missile Launcher. This means the enemy needs EIGHT anti-nuclear missle launchers constantly producing anti-nukes to counter one of these. The only downside is the massive time and resources required to build the launcher in the first place and the amount of energy it drains when producing one of its missiles.

I have played other RTS games like Starcraft or Company of Heroes, but this is one I regularly return to as the sheer scale is epic and it has some nice UI features that other games lack. UI features like the ability for a factory to repeat it's Que until stopped or for a transport to automatically ferry fresh recruits to the frontlines.

Here is one of my favorite videos to use to describe the sheer scale of the game. I was not kidding when I said each player had hundreds of units.
[youtube]https://www.youtube.com/watch?v=2SLK5twlgrg[/youtube]

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as far as Overrated games, that would be a matter of opinion on my part. I've played a fair amount of good games that I did not enjoy for one reason or another. The Witcher 3 is one example. Good story, Good amount of content, but the combat system felt god-awful to me. Played it once, beat it, have not touched it since. Even when the DLC came out (and I have the season pass as I generally trust in my judgement) I have not played it besides that one time.