An old RTS game, heads and shoulders above the new ones.
First of all, let me outline some of the elements within Total Annihilation that set it apart from other games when it was released and then discuss why it is still the best of its kind today.
Total Annihilation was the first game of its type to successfully utilize 3D environments. The terrain is filled with destructable objects such as trees that you can use to gather energy from or just let become part of the devastation of battle. The environment plays a big role in Total Annihilation. Gaining the high ground in a skirmish game and building a powerful defensive weapon can completely make the difference. The 3D world affects how missiles are projected and the graphics hold up, even by today's standards.
The scale of Total Annihilation is huge. The battles are constant, massive, and utterly epic. Every battle will leave behind scars in the form of a graveyard of wreckage, which can in turn be harvested to gather metal resources or destroyed by the next battle wave. The variety of weapons is also astounding. Even in the original, stand-alone game there was a plethora of options. I was always fond of the Cans and/or Lugers on land, and Brawlers in the air. My favorite defensive weapon is the Annihilator, which, as I mentioned before, is incredibly effective when placed upon a large hill with some defenders to protect it from air attacks.
Some of the maps are incredibly huge, especially considering the time it was released. You basically have the flexibility to create as epic of a matchup as you want. Amazingly, being able to select as many units as you wanted to wasn't a standard element of RTS games at this time, Warcraft and Starcraft being major culprits of this. Total Annihilation finally spared us of that ridiculous little roadblock, allowing you to select as many units as you could see. This is another little element that helps you realize the massive scale of the game.
Despite it's size and scale, Total Annihilation somehow manages to stay personal. Every unit under your control keeps a tally of how many kills it has. If it single-handedly gets over five, it gains veteran status. This small and seemingly irrelevant feature makes the game just a little more personal and it was something just not seen in RTS games at this time. You'll care for your veterans and probably want to see just how far their skills can be taken by keeping them alive longer than the average cannon fodder.
The resource system was quite different than most games at the time and gives the game a uniqueness unmatched until the release of Supreme Commander. In Total Annihilation, there are only two resources, metal and energy. Rather than gathering them and building up a stockhold to use later, you are expected to balance the constant amount you are using with the amount you are gathering (for example, you'd want to be gathering more metal, +20, than the amount you are currently using, -15). You can build storage buildings to store a greater amount and provide greater flexibility or padding for times of need, but it's not necessary for building a strong army and defense. However, if you don't keep a balance, you can run in to some serious problems. Running out of energy will severely inhibit your ability to build things, especially since your metal extractors require energy to work. More commonly, you'll run into issues of running out of metal, which is gathered at a slower rate. Keeping a balance between resources is absolutely key to winning the game, especially if you're playing another human opponent (who likely won't make the same mistake as you and won't hesitate to punish you).
Total Annihilation is very customizable and still has a huge, active community. If, despite my insistence that the graphics hold up well today, you're unconvinced and think it needs an upgrade, you can download graphical enhancements that will allow you to zoom in and out and change views while providing higher resolution and texturing. There is also an incredible amount of maps and new units that can be downloaded or created (for the adventurous type). I would also be willing to bet that you'll always be able to find a multiplayer counterpart, though I mostly stick to playing over LAN with friends. Large and active communities, especially for older games, are always a testament to a game that has transcended time.
For all the great things that Total Annihilation offers, I have two things to warn people about. One, I would recommend starting with vanilla Total Annihilation. There is so much content added with the expansions and especially with the downloadable units that you may be overwhelmed. As a seasoned Total Annihilation player, I was completely out of my league playing with a downloadable unit pack simply because I didn't know what units were effective and what buildings did what. Second, Total Annihilation is not about building a society (no Sim City elements). In other games, such as Age of Empires and games similar to it, your "civilization" really just serves as a way to advance, gather resources, and build a stronger army, making it not much different than Total Annihilation. However, those games do like to put an emphasis on building up a society even if it's nothing more than a red herring. In Total Annihilation, the focus is simply smiting your enemies with as much force as possible.
All of my friends that played Total Annihilation ten years ago still prefer to play it than firing up a game of its unofficial sequel Supreme Commander, which feels somehow less personal to them. Though it's hard to explain this phenomena completely, I tend to blame this on the tactical zooming feature - something is lost when you're not in the firefights up close and personal. Being a seasoned RTS fan having played everything from the Warcrafts, Rise of Nations, Starcrafts, Age of Empires, Command and Conquers, Supreme Commanders, Company of Heroes, and Empire Earths of the genre, I must say that Total Annilation stands head and shoulders above the rest.