Ifirit, Ramuh, and Leviathan are all the same fight. Only Moggle Mog (which comes from V1.0) and Titan (who was almost in 1.0) have significantly different mechanics in the ARR set. Thus you play three of them exactly the same way. Beat on the primal, add phase stop all combat on the primal to remove adds, get light-enrage animation or enrage, continue beating on them to second add phase, stop beating on the primal to clear second round of adds, then defeat primal.
Shiva isn't that different, only lacking the "static adds" phase. Moggle Mog 's add phase is where all the mechanics are. Titan on the other hand is the least forgiving. Titan also has an add phase on Extreme, which is why Titan has to be defeated quickly, otherwise there is no room in the arena avoid the puddles.
The Warring Triad basically has Shiva's setup, where there is an easy mode followed the "real fight".
Altogether I think Shiva, Ravana and Tsukuyomi 's music timing set all the moods for their phased fights well.
So I'd say the least favorite, mechanics-wise is still Titan. Design-wise I think Ramuh has the least interesting design as he's all beard.
