Actually, since Slowbro/Slowking can both learn Calm Mind and Amnesia, Slowbro is the best of the two in terms of pure tanking ability, for it's higher Defence stat. Bear in mind that both are part Psychic, so they do have a physical weakness, to Ghost moves. I consider Slowbro more of a tank, and Slowking more of a sweeper. It might not be a fast sweeper, but it's kind of like Parasect - it doesn't need to be fast.
And I'd rather look at actual base stats rather than some made up 1-4 scale, because that hardly tells you any difference, when 5 points between base stats means the difference between one Pokemon being faster for example.
For example, let's take that 1-4 scale and look at some top end Pokemon in the speed stat. All of these would be rated 4 in this scale:
Swellow
Dugtrio
Starmie
Aerodactyl
Jolteon
Now, can you tell me which is actually faster? If you just compare on a 1-4 scale, you would think that all of these Pokemon would have equal speed, but the difference is very subtle - but important in remembering who goes first. The actual order is:
Swellow - base speed 115
Dugtrio - base speed 120
Starmie - base speed 120
Aerodactyl - base speed 130
Jolteon - base speed 130
The point of this: learn the base stats, because knowing this is crucial in preserving your Pokemon long enough to win. Some made up 1-4 scale means nothing when a slight difference makes all the difference.
Rant mode off.
