There is no "best" pokemon in anything. All pokemon have their unique traits, one pokemon may surpass in one area while another pokemon surpasses in the other.
STEEL
| Forretress | Steelix | Skarmory | Heatran | Metagross | Scizor | Registeel |
Forretress - Makes a fantastic Physical Wall with that enormous Defense Stat. However, its x4 weakness to Fire limits its usage. Though it gains access to two important key moves - Rapid Spin and Spikes
Steelix - Has a skyrocketing Defense stat with a somewhat decent HP stat. It is one of the best physical walls around, though it is still weak to Earthquake - a major drawback
Skarmory - Is a great Physical Wall due to the huge Defense stat and the numerous resistances it gains. It also sets up Spikes and shuffles the team around. It gains an immunity to Ground and escapes Spikes and SR.
Heatran - is probably not the best Steel since it has a x4 weakness to Ground. Earthquake being one of the most popular moves on a physical sweeper. However it is still pretty versatile
Metagross - Is amazing at Sweeping while still being able to defend, although it requires one turn to set up and risks PHazers.
Scizor - is a good Baton Passer. However, Ninjask would accomplish this task much more thoroughly than Scizor.
Registeel - Is an extremely good dual wall, though it has numerous weaknesses to very common types. Its defenses are skyrocketing at 150 base, along with a decent HP stat that can certainly be frightening.
DRAGON
| Salamence | Dragonite | Garchomp |
Salamence - is quite unpredictable and gets through most pokemon since you never know if its going to be SpecsMence or PhysicalMence. Its coverage and movepool is quite large too, though its speed stat could use some work
Dragonite - can stand probably at the most of one Icebeam with the help of Yache Berry with its SDefense. Though it needs one turn to set of Dragon Dance, after that, it outruns and destroys most pokemon
Garchomp - Outruns Salamence and strikes with Dragon Claw and with the help of SR, its an OHKO. It's quite diverse and destroys the SkarmBliss.