Command & Conquer has always had an active modding community that has kept the classic RTS series relevant for far longer than its rightsholder who shall remain nameless tried to push it.
So here are our top 8 mods for any Command & Conquer game, sorted only alphabetically so we don't get too many rage messages ;)
Contra (for C&C: Generals - Zero Hour)
One of the oldest mods in active development, in production since 2004! While the unmodified Generals and Zero Hour keep high-tech units to more realistic levels, Contra goes beyond those restrictions and introduces mechs, cybernetic units, heavy weapon hovercrafts, weather manipulation devices akin to those in Red Alert 2, and more. How much more? Well, download it from here and find out!
Mental Omega APYR (for C&C: Red Alert 2 - Yuri's Revenge)
This mod has been in development since 2005, and with each major release, it came with a complete overhaul of not just the factions and their designs, but also the entire narrative. In the 3.x.x series, the storyline is basically a reboot of the entire Red Alert 2 narrative with intense campaigns for the Allies, Soviets, and Epsilon (known as Yuri's army in the unmodded game). In the most recent 3.3.x patches, a fourth faction of mysterious background called the Foehn is added, and it also gets its own mini-campaign. With insane map counts, completely overhauled armies, and an all-new soundtrack, Mental Omega could even be considered its own game, and its repeated presence in MOTY top 10's confirms its status as a must-play. Get it here!
Red Alert: A Path Beyond (for C&C: Renegade - standalone)
With exclusive permission given by EA Los Angeles over a decade ago, the team at W3D Hub has been releasing their Renegade mods as standalone games. Their flagship title is Red Alert: A Path Beyond, a multiplayer-only total conversion that brings the gameplay of Renegade's hailed multiplayer mode to the universe of the first Red Alert game. It started out as RenAlert nearly a decade and a half ago and it still gets updated, with even new units like the Chrono Tank debuting recently, and there are some organized game nights if you want to have a guaranteed entry to well-populated servers. This mod is available through W3D Hub's very own launcher that you can get from their site.
Rise of the Reds (for C&C: Generals - Zero Hour)
New generals and units in Zero Hour is not enough for you? How about a mod that adds two new factions, the Russian Federation and the European Continental Alliance? Rise of the Reds brings all that and more - a more defined technology tier system, a better air/anti-air counter system, redesigned original units, new special effects and... just try it out for yourself by getting it here.
ShockWave (for C&C: Generals - Zero Hour)
Any list without one of the most popular Zero Hour mods would be empty. ShockWave tries very hard to make a larger distinction between the generals than in the unmodded game, and brings new units that aim to maintain the design styles of the original armies. In addition, two generals who were tucked away in the game files are restored, and the Chinese boss general Leang is now playable as well, complete with General's Challenge mode support. Get the mod from here.
The Red Alert (for C&C: Red Alert 3)
Another faithful recreation of the original Red Alert, this time in Red Alert 3. This one misses out on nothing, as it has all the units and structures from the original game and its Aftermath expansion pack. Unit designs have been kept true to their ingame appearances from the original for maximum nostalgia. The team even managed to replace RA3's polarizing ore node system with classic ore fields! The release trailer for the mod is actually a replica of the original game's intro. However, the mod does not have any singleplayer campaigns, just offline skirmishes and multiplayer. You can get it here.
Tiberium Essence (for C&C 3: Tiberium Wars)
This mod is the result of the creator making his own rendition of a sequel to Tiberian Sun, including units that were no longer seen in Tiberium Wars and some completely new ones, including the much-neglected Forgotten faction. You can see why this is the most popular Tiberium Wars mod after you download it for yourself from here.
Twisted Insurrection (for C&C: Tiberian Sun - standalone)
Thanks to EA Los Angeles releasing Tiberian Sun as freeware in February 2010, the game's modders have begun making standalone games rather than mere mods. The most noteworthy among those is Twisted Insurrection. Its premise takes the Nod ending of the original Command & Conquer as its starting point instead of the canon GDI one, and builds up a whole new universe branch from there. The lore goes so deep that it's basically two mods in one - the Twisted Dawn phase of the campaign takes place immediately after the original C&C storyline and has more conventional weapons at your disposal, while the main Twisted Insurrection phase gets all the scifi gadgets. Has too many completely original music tracks to count, two of which were remixed by Frank Klepacki himself! You can download it here.