Infinite Undiscovery

Michelle Sigmund anyone other than Aya and Eugene

Spoiler

You can't use Edward or Aya in Vesplume Tower.

Capell ,Aya ,Edward ,Eugene . Beat the game with them

Party selection can be a pain. If you want to be sure to open all the chests you can come across without having to go back to much I recommend the following:
Capell, Balbagan, Komachi, Michelle

Balbagan is one of two characters who can break the cracked rocks, plus he inflicts a lot of damage.
Komachi can open locked/trapped chests as well as open chests from a distance.
Michelle has a trait that she always fights better with Capell/Sigmund plus she's an adept healer.

In response to using Gustav, I would only use him in your party if you are at a sufficiently high level that you wont require an out and out healer. Gustav is much more effective with Aya in the same party but because he takes up two slots you sacrifice having a mage. While Aya has some basic healing spells she is nowhere near as effective as Michelle or Eugene. It works in a sense as Gustav does horrendous damage to most enemies but if you chose to do this I recommend you have a lot of potions as back up.

They all have good strengths...

Any combination works, so pick your favorites!

My favourite team to use is Capell,touma (how ever u spell it) ,rico,michelle or eugene eugene learns revive and michelle doesnt so with michelle make sure u have a few MM's

Actually, there are some bonus which you can see in the personnal skill windows :

Sigmund gives a global +ATK+DEF+INT bonus.

Aya + Gustav =
Gustav +ATK+DEF+INT
Aya = -INT+ATK (If Gustav is badly hurt, she gain +INT+ATK+DEF)

Michelle + Capell
Michelle + (INT with Capell) + DEF
Capelle + ATK (if Michelle is in bad condition)

Some bonus stated before require you to do some secret conversation by unlocking Traits & Titles.

Unlike most who use Komachi, the best bet agains long battle (Boss fight) is Capelle+Balbagan+Vic+Eugene (like in SG)
Reason is :
- Balgagan take only 1 slot and can break rocks. (unlike Gustav with his huge 2 spaces) After each minute in fight, he gain +ATK+DEF bonus. He can become as strong as Gustav, if he fight for 5 minutes or more.
- Vic can unlock chest. It's sure that she can't open out-of-range chests, but she got the same bonus as Balgagan : Become more powerfull over time, while fighting, and while at high level, it can be quite a change. She can become as strong as Capelle after 5 minutes.
- Eugene can revive and he doesn't have any "weakness" (Aya=Bugs, Michelle=Birds, etc.) which affect some stats.

Capell - Auto.
Edward - Get Twinstream to level 3. It's godly.
Michelle - Get Astro to level 3. It's also Godly.
Komachi - Hornbeam is an excellent ranged attack.

This combination might seem weird, but almost every really tough boss fight near the end of clearing normal seemed to consist of these threeelements:
1. I did NOT attack with capell at ALL, because I was too busy using him to heal through the menu.
2. Edward did ALL my upfront attacking with repeated user-commanded Twinstream attacks.
3. Michelle and Komachi stayed far away and used their ranged attacks.

Edward,Eugene,Balbagan/Michelle. It's never done me wrong

You will always be stuck with Capelle, so early on in the game start levelling up his battle skills (just use them ALOT). Keeping that in mind:

Capelle (with Grinn Valesti lvl3 you can OHKO anything ;P)
Balbagon (Blitz Brigade lvl3 = funny and devastating !!!)
Komachi (Magidex has the right idea : hornbeam lvl3 ftw)
Eugene (sounds weird? no, hes actually the best mage in game)

honestly is doesnt matter what team you use if they have level 3 skills you force them to use, I cannot stress how awesome it is.

During gameplay, my party was generally w/e was available the first half of the story. After that,

Second Half -
Capell - Required. GV anyone?
Edward - Overpowered. Hell, even the game calls him overpowered in his traits.
Touma - Fast, strong, and generally just fun to watch kill. There might be better options here, but I just put him in once and then kept using him.
Eugene - Best mage in the game hands down. I have serious issues with all the others.

Post Game/Seraphic Gate -
Capell - Again, required. Grinn still rapes though.
Edward - See above. Even more so with Seraphic crafted weapons.
Sigmund - Having him GV at almost the exact same time as me, just makes me laugh with joy.
Eugene - See above. Plus, a healer is always nice. I get tired of using items.

Balbagan also does well Post-Game, though his top weapon having a life drain does annoy me a little.

Dungeons - Making sure nothing is missed
Capell - Required
Balbagan/Gustav - Those damned rocks.
Komachi - Honeysuckle and locked chests.
(If Balbagan)Random Mage.

Capell ,
Aya,
Ed,
Eugene.

I solo with Ed and Capell.