Beware! Spoilers below, possibly not hidden!
This is all subjective and personal, and depends on playstile. I typically manage to do no grind runs (no Bai Ze or landworms grind at all), and still take absolute conqueror down with not much trouble. Picking decent leaders is the key. Now what makes the leader decent? Arts available and the final item set. Non-unique arts in the order of decreasing importance: Revives (Herbs, Remedies etc)->Wards->Hexes->Psi. Combat arts are available to any character, so they are not listed.
Typical setup usually includes one Cache+aoe union (ex: David-Zolean-Yuniver-Caedmon-Nora, Crossbow III; Gaebolg, Crimson flare, Cache, silence immunity, 2 healers - or even 3, if Nora manages to train lotions enough) and 4 melee unions with revives and wards. Melees do better damage against single targets, and thats what the bosses are.
Oh, and i play it on pc.
Baulson. Pros: Remedies+Herbs - solid healer options, ends up with the highest possible Atk weapon in game, available very early, so can get a lot of training if on the first run, good Parameter Bonus. Cons: very poor Formation Attributes, on NG+ you can get much better (statwise at least) leaders in almost no-time, so he falls behind. Summary: a solid hp spounge pick on the 1st run, ok for NG+ if you manage to fit him in a formation
Blocter. Pros: Herbs, CA+5 final weapon, will most likely end up Scout (cheap arts), Wonder Bangle. Cons: none. Summary: solid pick, especially on the first run, since most other sources of Debuff Duration arent available yet.
Dave. Pros: Gae-Bolg will bring you thru any major non-boss battle, Herbs+remedies, can easily end up an Assasin with WA, has shield. Cons: terrible, terrible AP, which grows terribly slow (at least in all my runs). Summary: Almost a-must union leader until after Koenigsdorf, a solid pick for shieldbearer union after.
Pagus. Pros: Remedies+Herbs+Wards, ends up with +15% int Remnant weapon. Cons: none. Summary: very solid pick, can be either a good MA user, or a good CA user with his heartache. And wards.
Torgal. Pros: Sovani. They typically tend to do more melee damage somehow. Can have WA. Psi. Cons: no heals. Summary: Quad-wield, Combat Focus, make sure other folks in his union can heal. Solid pick.
Loki. Pros: CA/MA+5 final weapon, can learn hexes and herbs. Cons: nothing special at the moment you get him, you will have a lot better options when he decides to get useful stuff. Summary: since you will need him anyway for a quest, a possible placeholder in early fights.
Caedmon. Pros: Sovani. Herbs, later remedies, ends up with +5-to-itself weapons, decent Parameter Bonus. Cons: none. Summary: No Special attribute, required hire for a quest chain, the only Sovani that can end up MA user without wasting too much combat usefullness.
Gabriel. Pros: herbs, later lotions (if you can handle the pain), otherwise basically Dave - possible Assasin with WA and shield, even more speed from Parameter bonus. Cons: none. Summary: solid pick for shieldbearer squad.
Nora. Pros: possible lotions (pain), possible hexes, Flâchonelle. Cons: no heals, late (but solid) parameter unlock. Summary: No special. You really want that silence immunity amongst your MA union, if you can handle her attribute.
Leshau. Pros: Hexes, Psi, awesome Parameter bonus. Cons: no heals, years of training for any MAs. Summary: there are better MA users out there.
Maddox. Pros: hexes, possible herbs and remedies. Cons: Years of training for everything, tries to steal Pagus' Remnant ^^. Summary: better than Leshau, still not worth it.
Rhagoh. Pros: +5 MA/CA weapon, Remedies+lotions(pain). Cons: see Loki above. Summary: see Loki above.
Jorgen. Aristocrat has Wards, so i recommend aiming for him. Pros: Wards, herbs, lotions(pain). Decent Parameter bonus. Cons: late recruit, wierd weapon choice. Summary: Wards make any character a solid pick.
Darien. Pros: Herbs+remedies+lotions already at usable level! Instakill weapon. Solid Parameter bonus. Cheap arts Cons: none. Summary: a solid pick.
Glenys, Pros: Remedies+herbs. Cons: closes a spot for a grind trick. Summary: there are better picks, but nothing wrong with her.
Emmy. Pros: remedies, UA. Can end up assasin with WA, but that will require ridiculous amount of effort and also cripple her healing powers. Cons: none Summary: a solid pick as a union leader, at least until 7 are available.
Roberto. Pros: RWA Cons: no heals Summary: there are better union leaders.
Gaou. Pros: Lotions! Rwa, possible hexes, final weapon+Parameter = a lot of Str. Cons: none. Summary: a very solid pick even as a not union leader.
Allan. Pros: Sovani, WA Cons: no heals. Summary: basically a Torgal, but without UA.
to be continued