If we are going to do the mathz, Alistar's hard CC lasts up to 2.5 seconds and only 0.5 of that is affected by Tenacity. More over, part of it is displacement and the other part is AoE. I don't think there's a more devastating combo, but I don't see Nautilus a lot so I could be wrong.
Leona and Alistar are the best CC machines on the fact of chaining CC and cooldowns.
Nautlius technically has the most CC available to him, but he can be similar to someone like Udyr in which sure, you pull off your CC, but what do you do while you're waiting for the timer to run down before you do it again? You're just a meatstick.
Unlike Leona/Alistar who will be able to CC you again in just a matter of seconds.