Which Titan has the best Ultimate Transformation?
Gameplay wise, whose ult is more valuable?

Gameplay wise, whose ult is more valuable?

I think you get the most value out of Nasus'. The bonus cdr on his q, which is his main form of damage is hard to beat.

From a power standpoint, Nasus probably takes the cake. Increasing amounts of armor / magic resistance per second, increased health, increased attack range, AoE percent max health damage, AND it halves the cooldown of his primary damage source.
Aatrox gives him 25 percent increased AD, a burst of movement speed, and amped healing. Granted he heals insane amounts, he's still susceptible to being bursted and killed before he actually gets to utilize it.
Renekton is basically just an AoE aura with increased fury generation, and a max health increase. Oh and a 25 range increase.
Shyvana's is good if only because of how people abuse AP on her. Not to downplay it, but I'm going to wager that she gets a rework since this playstyle doesn't really enforce what they envisioned for her.
Renekton's ult is very powerful on him for reasons that synergize with his kit (fury gain, HP buff for all-ins, animation cancel with it) but Nasus ult is the most stacked ability out of them. It just gives more than the others. Lower cooldown Q, resistances, HP, attack range, more damage than renekton aura, it's just got it all.