chaosblade said:
Pretty much what Bel Marduk said.
Shulk/Dunban/7th is the best overall damage output party if you're set up right. Swapping in Melia is good for the ether damage and buffs when necessary. Not to mention her quick topple.
There are also instances where Reyn is useful, since he's the best tank when it comes to fighting many enemies at once, but he falls behind Dunban and 7th against single enemies since he'll eat a lot more damage and require healing. He also deals less damage.
Sharla also sees uses in some of those bigger fights as well, I'd say when you use Reyn she should probably tag along too since he'll take a good amount of damage. I took her along for story fights too.
I never really used Riki, so I can't speak for him.
I used Riki a bunch mid-game as mostly player controlled, but because the Monado is so helpful in the last third of the game he kept being left out. He's still a great Tank though, because of his incredibly high HP (I keep it at 9,999, even if it's over kill, lulz), area healing and all those status attacks. I'm not sure he's the smartest CPU though. He'll heal when required, but sometimes it seems like he doesn't Poison / Bleed the enemy enough, while he loves to use his Topple attack. Tons of fun to use as Player controlled character though.
Shulk / Sharla / Dunban is what I've been using for most of the story fights now, with 7th coming in to build up affinity while I'm just running around. I appreciate CPU Sharla's diverse healing. The fights might be slower because of less damage over, but I feel much safer with Sharla in the party.
I do try to use everyone though when I'm in a comfortable area, like going back to do side quest. Really need to get that affinity up to see those Heart to Hearts.