In short i will be listing Rikku's most useful Overdrives and what Items i prefer to use to make them and how to get said Items.

Note: that many of the Overdrives listed have more Items combinations to make them than listed.
I only put those that are easiest and smartest to use here.

Chaos Grenade:
7,000 Damage And Most Negative Status Effects To All Foes.
This is a stupid good move until you get Break Damage Limit.
Any Grenade Item + HP/MP Sphere
I like to use HP or MP Sphere here as you dont need more than 9999 HP and 999 MP to beat any content in this game
You should get plenty of any of the items needed for this by playing the game normally.


Black Hole:
Reduces HP Down To 1/16th
You need Break Damage Limit to do more than 9,999 damage with it, also some foes are immune to it.
Shadow Gem + Gambler's Spirit
Shadow Gem are Normal Steal from Sand Worm in Sanubia Desert, you also get 99x for catching each of the fiends in Bikanel.
Gambler's Spirit are normal Steal from All Original Monsters in the Arena.


Final Elixir:
Heals All HP/MP, And Most Status Effects + Revives Allies From KO.
Does Not Remove Breaks, Zombie And Doom.
Any Level Key Sphere + Mega-Potion
You can also use Hi-Potion and X-Potion, i prefer Mega-Potion because i tend to use Auto-Potion Endgame. But if you dont use Auto-Potion then use Hi-Potion.
Mega-Potions are easy to get by bribing (237,500 Gil) Grendel's in Mt. Gagazet: x60 Mega-Potions.
Lvl-1 Key Sphere's are drops from Master Coeurl in Omega Ruins and Rare Steals from Earth Eater in the Arena.


Ultra Null All:
Cast Null All + 5x Cheer, 5x Aim, 5x Focus, 5x Reflex and 5x Luck On All Party Members.
Healing Spring + Healing Spring
Healing Spring's are drop from Kottos in the Arena.


Hyper Null All:
Cast Null All + 5x Cheer and 5xFocus On All Party Members.
Musk + Musk
Hypello Potion + Hypello Potion
Musk + Hypello Potion
Musk are Rare Steal from Evil Eye in Lake Macalania.
Hypello Potion are Rare Steal from Mafdet in Lake Macalania.


Hyper Mighty G
Cast Protect, Shell, Haste, Regen And Auto-Life On All Party Members.
This is a super OP move because of the Auto-Life.
Gambler's Sprit + Healing Spring
You can also use a Light, Luna or Star Curtain instead of a Healing Spring.
Gambler's Spirit are normal Steal from All Original Monsters in the Arena.
Healing Spring's are drop from Kottos in the Arena.


Hot Spurs:
Overdrive Charge Rate +50% To All Party Members.
It Stacks With Eccentric, can also be used as a substitute for Double/Triple Overdrive on gear for the Don Tonberry AP Trick.
Twin Stars + Shining Gem
Unlock Jumbo Flan at the Arena for 60x Twin Stars.
Shining Gem you get 60x for catching each of the fiends in Lake Macalania.


Eccentric:
Overdrive Charge Rate +100% To All Party Members.
It Stacks With Hot Spurs, can also be used as a substitute for Double/Triple Overdrive on gear for the Don Tonberry AP Trick.
Twin Stars + Gambler's Spirit
Unlock Jumbo Flan at the Arena for 60x Twin Stars.
Gambler's Spirit are normal Steal from All Original Monsters in the Arena.


Trio of 9999
All Actions Do 9,999 Damage or Healing.
Its good early on, but i dont use this much.
Gambler's Spirit + Gambler's Spirit
Shining Gem + Gambler's Spirit
Gambler's Spirit are normal Steal from All Original Monsters in the Arena.
Shining Gem you get 60x for catching each of the fiends in Lake Macalania.