Are we talking about the look and design or the gameplay mechanic?

If we are talking about gameplay, shiva for me i guess, the whole fight is just your standard fight, there is no unique mechanic from it, it just feel.... plain. King moggle fight is actually the number one for me, but the whole thing seems like a big joke to me (you know since its... moggle...) so i give it a pass.

Best one is definitely titan for me, still the one that give me the most "impact" when i first fight him, the mechanic, the music, the chaos, holy wow for the first time.

As for look and design, ifrit, everyone look humanoid or at least has a "shape" for it, ifrit just straight "monster" design for me, its really bad.

Best one is par between garuda and bahamut. Bahamut is pretty iconic so you cant go wrong with him, as for garuda... love the design and the cutscene seeing her first time is really awesome.