The Scorpion is the main tank used by human forces in the Halo series. With it's powerful turret blasts, it's by far the ultimate vehicle in the series.
In multiplayer nobody passes the chance to use it. If it's known that one's on the map, you'll see a big race for it. It's take a lot of skill and some luck to take a Scorpion down. The only weakness is a slow rate of fire, and sluggish speed.
I was glad to see it back in Halo 2. Though the lock-on rocket launchers made it slightly less invincible.
I still loved using it in Halo 3. Had my biggest win streak ever using one.
I was sad that they're hard to find in Halo: Reach. There's so many vehicles that seeing one is more of a rarity.