Speaking of theorymonning, Arpa and I also daydreamed up a Poison/Steel legendary designed with Ubers in mind. The ideas of disease slowly eating away at its foes and other delightful concepts came to mind, so we figured a very bulky tank would work quite nicely. Thus, Titarnius was born and it is a complete beast.
Titarnius
Poison/Steel
Contaminate (Eliminates Terrain effects)
155/110/145/90/140/40
We came up with a rather vague moveset consisting of moves like Stealth Rock, Earthquake, and other moves that befit its role as a tank. A signature move came to mind, also giving it a rather deadly role as a trapper that can help guarantee safe switches and just wear the opponent down without a problem.
Attrition
15 BP Physical Poison
10 PP
Traps for 4-5 turns, deals 1/8 health each turn, initial hit has 30% chance of badly poisoning target.
Let's be practical and give some numbers to just how horrifying this thing would be to face in a battle, shall we?
tl;dr it walls four out of six of Ubers' S-rank threats, traps them with Attrition, and then just pesters them to death.