Probably Ignis and Gladiolus. I only saw one attack from Ignis so far, but it was a quick lunging sword strike and apparently he has a lot of support abilities too. So far that sounds like a pretty decent character. Quick attacks, and keeping everyone dealing better damage and taking less. Then you have Gladiolus, a huge meat sponge for big moves and drawing the Behemoth attacks while Noctis focuses on dishing out the damage.
At the moment I don't anticipate I'm going to really like Prompto. --- "I am the Truest Repairman!"
Ignis and Cor because of their katanas (but I doubt they will be available at the same time frame). As for a third choice I would go with Stella (if she ever joins the party) or someone else that uses a thrust sword (rapier), katana or bow.
Ignis principally. Also Cor, and if he bites it, then Gladiolus. For many of the reasons already stated. Ignis is the cool, calm one; and uses a katana. There's no other character I'd rather play.
That said, I will make a concerted effort to play as Noctis and Prompto as little as I possibly can. Surely we'll be forced to play as Noctis early on, but when the others join, hopefully we can play 100% as other characters all the way up till the very last battle.