The DeeZire mod is by far the most well known and is in the opinion of many, the very best mod for Red Alert 2.
DeeZire adds new features, units and structures, and customizes each nation to where you will want to consider more than just the bonus unit. Each nation now has more than just one bonus unit, and they also get different price benefits for different units and structures.
For example, the Cubans now have the Hind attack/transport helicopter, and the Terrorist now can disguise itself like a Spy. The Russians get the new cluster-missile armed Oktober Sub. The British Phase Tank can disguise itself as any terrain object. The German Paradox Tank fires a sonic shock wave that causes enemy infantry's bodies to shatter. The Allied A-10 Warthog does strafing runs with napalm canisters, and the Soviets gain the Parabombs, as well as a new tech level, spawned from the Soviet Tech Center. And not to forget the addition of the one and only Soviet Super-Soldier: Volkov!
And that's just some of the new units. The new and enhanced AI utilizes new units, tactics, and actually acts as more of a challenge than a newbie. The new AI is greatly improved. Each country will play to it's own strengths all while making sure to guard against their own weaknesses and handicaps. The computerized enemy will scout the map and your base, establish waypoints, repair bridges as well as blow them up, replace destroyed Construction Yards, repair vehicles, shut down radar, guard Tech Buildings and more! The vast improvement in the computer's intelligence is startling and hard to predict.
As in previous Command and Conquer games, you can now sell vehicles that are being repaired at the Service Depot, allowing you to get rid of extra mind-controlled vehicles for a tidy profit. There is now the "DeeZire Boot Camp" mission available upon completion of the Boot Camp tutorial. It allows you to use all of the units from the game, letting new players get acquainted with the way Red Alert 2 works. There are also expansions that you can download off of the official DeeZire web site. The first expansion allows you to hear three song themes that were cut out of the final version of the game, as well as adding some new loading screens. The second expansion lets you play the infamous Giant Ant missions from the original Red Alert game, including the movies! And the third (and my personal opinion, coolest) expansion replaces the default taunts with taunts and other voices from previous Command and Conquer games. Now when you play LAN, you won't hear General Vladimir complain about his resources. Instead, you will hear CABAAL tell you that you should "Observe superior tactics, while you still have human eyes."
DeeZire also adds many new modes of play. You can now play Naval War, Meat Grinder, etc. in Skirmish mode. The mod also adds several new modes, such as Crate War and Global Dominance, in which you get the bonus units from all of your side's countries, i.e. an Iraqi player would have technology from Cuba, Russia, and Libya, as well as their normal technology. Several maps that were either damaged or never released in the original game are restored and ready for play, giving players a total of 40 new maps for play. There is a new Tech Structure, the Tech Armory, which upgrades any infantry unit you send into it to Elite status.
The DeeZire mod brings back great things from games past, and introduces things new. With tons of new features and units, DeeZire might just help ease the "pre X-pack" anxiety cases that are increasing throughout the Red Alert 2 community.