Really, these are the only ones I generally stick with in FF games.

VI-Terra, Celes, Sabin, Cyan (Two heavy-hitters and two great spellcasters).

IX-Zidane, Freya, Vivi, and Quina (Very versatile. You have two hard-hitters, two people who can heal and support, and two spellcasters. Quina can fill most areas, Freya can deal insane damage while having good support abilities and HP, Vivi has his spells, and Zidane can deal damage while also protecting others with his Protect Girls and Freya's respective Cover.)

X-Tidus, Auron, and Rikku. Tidus is fast, Auron deals and takes damage, RIkku can serve as a surrogate healer/spellcaster with het items.)

XIII-Lightning, Sazh, and Vanille. (Covers all six major paradigms, Sazh and Light can do the attacking, Lightning and Vanille can heal, Vanille and Sazh can support, and all three are decent Ravagers.)

I tend to prefer versatility to specialization.