Total Annihilation is the absolute embodiment of everything RTS stands for. Comparing videogames to board games Starcraft and Warcraft are akin to Chess and Checkers, while Total Annihilation would be RISK. Vast amounts of decently balanced units, huge maps with 3D terrain, and great physics that actually have an impact on strategy make for long drawn out battles of epic proportions. Nothing will get your heart pumping more than watching a Krogoth bot lumbering towards your base as your artillery rains down fire from the hills above, knowing the inevitable outcome. Maybe you'll have time to evacuate your commander and rebuild again in some remote portion of the map but then again, maybe not.
Another great aspect of TA is the simplicity. My father (now in his 50's) still plays TA and often pauses the game overnight to come back to the battle the next day. No other game has the scale and yet maintains the simplicity that allows young and old alike to feel like actual battlefield Commanders. The interface is exceptional, clean, and again simplistic (in the best sort of way). Queuing orders is amazingly simple yet powerful. Winning is not how quick you can click the mouse or keyboard, it's how imaginative you are. Actual battlefield tactics work, for example reading up on Sun Tzu might do you very well in multi-player battles.
The story is nearly non-existent, yet it manages to be adequate. TA is not about the story; it's about surviving and conquering. It's not about the latest gee-whiz graphics, it's about function and imagination.
To date no one has been able to touch TA for shear RTS goodness. Supreme Commander tried, but ultimately fell short (and many would say very short) of it's spiritual predecessor. Do yourself a favor and try TA! This is, without a doubt, one of the greatest games of all time.