Ranking
Original Post
Warzone 2100
WZ2100 is an old RTS game made by Pumpkin Studios which originally came out in 1999. Unlike most RTS games, where units are unlocked in a linear path, Warzone was unique in the sense that you were able to design your own units from a myriad of various parts, allowing for a total of 70,000 possible units- everything from construction trucks to VTOL craft armed with plasma cannons.

The story is essentially as follows: Warzone takes place near the end of the 21st century. Most of humanity has been destroyed by nuclear war, and much technology was lost in the process. You are the commander of "The Project", an effort to restore our technological horizons through exploration and research. Your primary foe, early on, are Scavengers- these are groups of ragtag, second-rate hostiles that are weaker than you from the get-go, but make it up for that physical weakness with sheer number and defensive structures. Many Scavenger bases contain technological Artifacts- by destroying (or as the game puts it, eradicating) a Scavenger base and collecting an Artifact, you can then reverse-engineer that Artifact by using a Research Center and then be able to produce it en masse, whether it is a defensive structure, weapon turret, or a new method of propulsion.

Besides the primary Campaign, there are also Challenges, side missions you can undertake that are meant to test your skill as a commander; the popular Skirmish mode, with up to 8 factions on a single map, and many settings for the mode such as the intelligence of each faction, whether or not alliances can be made, how advanced your base is when you start, etc; and finally, Multiplayer mode, which is essentially just Skirmish with human players.

There's a good chunk of user-made mods and maps available here; the game is completely free and can be downloaded here.

PROS
-Free
-Unique design system
-Varied gameplay
-Re-playability value
-Usermade Mods
-Very low system requirements, so there's no worries about whether or not you can run it.

CONS
-Not much official support: the game itself was "revived" in 2005 by a third party group of fans, and the original company is dead AFAIK.
-Graphics may seem lacking in comparison to other RTS games today, but they're bearable and can even pass for 'nice' on a high resolution.
-Friendly AI can be a bit wonky at times due to the mediocre pathfinding: sometimes units will bump into each other or move out the way, etc.
-Most missions have a set time limit to accomplish, usually an hour from what I've seen.
How to complain in style: GG, Mahulk.