Hardest: Safer Sephiroth.
Easiest: Tough choice between Ultimecia and Necron. But okay, Necron is the easiest. XD
Variety: Ultimecia.
Strongest attacks: Safer Sephiroth.
Most effeciant: Ultimecia.
Longest living: Safer Sephiroth.

However, if Trance Kuja were the Final Boss, instead Necron...

Hardest: Trance Kuja. I know you all will call me crazy because I think Ultimecia and Necron were easy and Trance Kuja and Safer Sephiroth were hard. I'll answer with two words. FLARE STAR. XD
Easiest: Ultimecia.
Variety: Ultimecia.
Strongest attacks: Trance Kuja. Flare Star. -_-"
Most effeciant: Ultimecia.
Longest living: Trance Kuja.