You can read around to find out. There's a lot of details about it...
Earth outclassed by Basalt (Defense)
Earth outclassed by Hell (Stamina)
Basalt outclassed by Duo (Defense)
Phantom = Duo (Stamina)
Duo somewhat outclassed by Revizer Revizer OR Revizer Dragooon Synchrom (Defense)
Detailed explanation-
Earth was the first top tier Defense and Stamina Metal Wheel.
Basalt outclassed it upon release, due to its sheer weight.
Hell + BD145 beat Earth to the top spot in the Stamina top tier list.
Then came various wheels in the 4D series, most notably Death, Phantom, Duo, and Diablo.
Death and Duo outclassed Basalt, due to their amazing low-recoil Defense. Duo soon proved itself superior and outclassed Death.
Phantom is an exceptional Stamina wheel, and thus holds the top spot in the Tier List. However, its tied with Duo, which is just as good. At times, Duo is considered better due to its superb Defense.
Diablo is the heaviest Metal Wheel (ignoring the Synchroms) which doesn't have much use in Defense due to its aggressive design. However, it is a formidable anti-attack wheel. It remained the most popular choice in that category; until Duo and Death surpassed it (in a way). It still is a good choice, no doubt about that.
Then came the Synchroms, which due to their weight, easily surpassed these Defense wheels. Revizer, which was a low-recoil wheel like Duo, outclassed the latter(when used as a Revizer Revizer Synchrom). The arrival of Dragooon was the final nail into the usability of the HWS and 4D Metal Wheels. Now its just Zero G wheels which hold the top spot in the Defense categories. For Stamina though, I don't think there's been anything new- except for the amazing spin-stealing Dragooon combo...
Team IB Revolutions
Thanks to N0body for the avatar and Synth for the signature!