Ultima is the strongest for most stats, but since you can only put it on one stat, either Mag or Str is the best choice depending on the character.
HP: Full-Life
You can draw Full-Life from Tonberry King and Bahamut. It can also be refined from Regen Rings (won from Torama's and Mesmerizes) with Siren's L Mag-RF. You need 10 Chimera cards for 1 Regen Ring so that's not the best choice.
Str: Ultima/Meteor
I would suggest refining Ultima from Dark Matter. You need 100 Curse Spikes, then refine those into a Dark Matter with Lvl. 100 Siren's Tool-RF. Defeat or mug Tri-Faces for Curse Spikes, they spawn at fixed points in the Deep Sea Research Center. You can also mod Tri-Face cards.
The easiest way to get Meteor is to draw it from Ruby Dragons. KO one party member before you fight one though. It won't hurt you then.
Vit: Meltdown
Draw Meltdown from Gaylas around Trabia and mod Gayla cards for Mystery Fluids which refine into Meltdown with Diablos's ST Mag-RF.
Mag: Ultima/Triple
Triples can be drawn from Odin and Cerberus. Other than that you'll have to mod a card which isn't available until Esthar.
Spr: Reflect
Reflect can be drawn from Adamantoises and refined from Dragon Skins which are won from Anacondaurs. You can also mod Carbuncle's card for 3 Glow Curtains, which refine into 100 Reflects each. You can win the Carbuncle card from Heart (
(SPOILER)Xu), who is a member of the CC group.
Spd: Triple/Haste
Draw Haste from Vysages around Winhill. You can also mod Kiros's card for 3 Accelerators. 1 Accelerator refines into 100 Haste. To get Kiros's card you must first lose MiniMog to the Queen of Cards. Talk to her in Dollet and she will tell you her father made the Kiros card based off of MiniMog. Then go to Deling City and challenge the guy wearing black on the left side of the street by the Shopping Arcade.
Eva: Tornado
Draw from Thrustaevis around Deling City. You can also refine Windmills. Thrustaevis and Abyss Worms drop those. Modding Abyss Worm and Pandemona cards works too.
Hit: Double
Double can be drawn from Grendel, Elnoyle, Cerberus, and Odin along with several other enemies.
Luck: Aura
Buy Hypno Crowns and Power Wrists from one of the Esthar shops, and refine those into Aura Stones with Siren's Tool-RF. Refine the Aura Stones into Auras with Leviathan's Supt Mag-RF.