My submission for "best possible party," for Hard difficulty, assuming a finish at level 20. I'm going to assume no Teller of Secrets books, for if/when the every-level thing is patched.
Dexterity +6 (11 base)
Constitution +2 (7 base)
Speed +2 (7 base)
Perception +5 (10 base)
Man-at-Arms 5
Expert Marksman 5
Tenebrium 4
Barter 2
Charisma 2
Scoundrel 1
Sneaking 1
Loremaster 1
Pet Pal
Guerilla
Picture of Health
Glass Cannon
Weather the Storm
Quickdraw
Bully
Craft Tormented Soul, Sextant, and Tenebrium Bar onto your Bow. Try to get another +1 Strength from gear for a 100% Knockdown Crushing Fist, but if you don't you'll live. Keep a sets of Bartering, Charisma, and Loremaster gear handy. Get Sneaking 5 off of Cautious trait and gear bonuses with your combat gear.
Strength +6 (11 base)
Intelligence +5 (10 base)
Constitution +4 (9 base)
Man-at-Arms 5
Aerotheurge 5
Pyrotechnic 4
Shield Specialist 2
Hydrosophist 1
Witchcraft 1
Scoundrel 1
Expert Marksman 1
Far Out Man
Leech
Picture of Health
Glass Cannon
Weather the Storm
Lightning Rod
Elemental Affinity
Craft Tormented Soul and Tenebrium Bar onto a crafted Sword of the Planets. You don't need more than 13 Strength to use a level 20 version of that sword plus 20 resist-all plate armour. Try to get another +1 Dexterity from gear for a 100% Haste on Fast Track, but if you don't you'll live. Fire resist is pivotal for healing off of your own Explodes. Tactical Retreat is your friend.
Strength +9 (14 base)
Constitution +4 (9 base)
Speed +2 (7 base)
Man-at-Arms 5
Leadership 5
Tenebrium 3
Two-Handed 2
Bodybuilding 2
Willpower 2
Scoundrel 1
Expert Marksman 1
Opportunist
What a Rush
Comeback Kid
Picture of Health
Glass Cannon
Weather the Storm
Leech
Craft Tormented Soul and Tenebrium Bar onto a crafted two-handed axe. Try to get another +1 Dexterity from gear for a 100% Haste on Fast Track, but if you don't you'll live. Fire resist is pivotal for healing off of Roderick's Explodes, which you will often be near. Try to get +1 Leadership on your helmet. Tactical Retreat is your friend.
Intelligence +10 (15 base)
Constitution +3 (8 base)
Speed +2 (7 base)
Man-at-Arms 5
Hydrosophist 4
Geomancer 3
Aerotheurge 3
Witchcraft 2
Blacksmithing 2
Crafting 2
Willpower 1
Far Out Man
Elemental Affinity
Scientist
Picture of Health
Glass Cannon
Weather the Storm
[free talent]
Keep Blacksmithing and Crafting gear handy, but wear more useful stuff into battle.
TL;DR: Glass Cannon, Picture of Health, and Weather the Storm are overpowered.
I feel one common error here is to try to undercut Str/Dex requirements too much, saying "I'll just get the bonus off of gear." You can also get Con/Spd/Per bonuses off of gear, so it's better to just use attribute points and guaranteed crafts like Tortured Soul to hit gear reqs, then you can choose between +Str/Dex gear or +Con/Spd gear, whichever you happen to find.
