why would karthus build sopur squishy vs rammus in a 1v1 battle?
i would go RoA, deathcap, void staff, WoTa, 20 stacks mejai, sorc boots. GOODLUCK RAMMUS

Tank is a word I wish we'd avoided, because "pure tanks" are generally characters that can initiate and soak damage, where I think some players refer to tanks as tough characters with no damage. I think tank examples are;

Rammus
Singed
Shen
Amumu
1/2 of Jarvan builds

It's the initiation factor and ability to cause team fights that is key. Leona will definitely be in that sort of role. I also think their damage is slow and eventual, as opposed to bursty.

Most older tanks are disable tanks (disables are what make them threatening enough for their durability to matter) and more of the newer tanks are damage tanks (Their combination of survivability and respectable damage are what make them threatening enough for their durability to matter). Damage tanks are not the same as "Tanky DPS", Garen although underwhelming is probably the best example of a damage tank.

While someone like shen I would call more of a balanced tank that has decent offense and disabling capability with some utility thrown in with shield/minor healing.

Rammus is all about singling out a target, his whole kit is around just ****ing up someone's day. Powerball catches up and slows them, taunt lasts 3 seconds and lets him drag them around, lowers their armor so his increased damage from his passive (boosted with DBC) hits harder, DBC returns damage on their autoattack which they're forced to do because they're taunted, and tremors is a strong AoE that they're standing in because of taunt and the slow from powerball.

Rammus is a **** when it comes to 1v1s, he's designed that way, throw one person in front to intercept powerball and give Cait a LW and some lifesteal and Rammus is dead.