Generally combo but that depends on the combo....
I personally use 3 rare 3 star ninjas since they make an awesome combo, but if it comes down to similar powered common ninja? Stars matter a bit.
But if there is a common ninja good enough to use, u'd probably farm it to 5 star, hinata, kimmimaro are some of the best examples, they are good and thus most player have them at 5 star.

best combo, of course. i hav gnw kankuro still 2 stard, but he really helps with my team of sasuke and hinata with my lightning main. It would b better 2 hav it combo based.

it's not the stars or the combo that are the most important but the synergy of your team.

I have a gnw kankuro but ı dont using him im now 63 lvl Im using hinata+main(lightning)+karin+sasuke 3 star my summon is ninja dog shiba and my chase is lightning sword (3.chase) is it good? Or Can do better than this combo ( I dont have much ninja)

Even Mu 2 star has an exceptional growth, i would prefer how the team synergize with each other over the stars.

I prefer combo + many CC