Say you could choose one between ALL the pairs of Mangekyō Sharingan to keep, and wield in battle (all Mangekyō's will be their respective eternal ones).
Itachi has shown proficiency with three techniques; Amaterasu, Tsukuyomi, and Susanoo (which wields the Sword of Totsuka and the Mirror of Yata).
Sasuke narrows it down to Amaterasu and Susanoo, while his manipulation with the flames is far beyond what we've seen from Itachi, through Blaze Release, and has even show the two being used in conjunction effectively.
And well Madara, his pair is pretty much his Susanoo, which is very powerful and large.
Obito has Kamui, and a winged Perfect Susanoo.
And there's also Indra's and Shisui (the latter's Kotoamatsukami cannot be spammed though, and still would have its previous cooldown).