Only two HP Pluses will actually work, which is plenty if they're mastered (both at +50% makes +100%...DOUBLES YOUR HP! ). Same goes with all the "[stat] Plus" materias. Doesn't work with things like Gil Plus or EXP Plus, though (only one is allowed, and when it comes to EXP Plus, only one per character). You can equip all you want, but only the allowed number will actually work.

(assuming they're all MASTERed and complete)
Weapon: Master Summon=MP Absorb, Master Magic=Quadra Magic, Master Command=HP Absorb, E. Skill=HP Plus
Armor: W-Item=W-Summon, W-Magic=Counterattack, MP Plus=Mega All (, Luck Plus=Speed Plus)

This gives you access to EVERY Command, Magic, Summon, and E. Skill. It also lets you get some MP back when summoning, use any magic four times at the cost of one, recover a little HP when using any attacking command (even just a regular attack), boost your HP by 50%, counterattack when hit, boost MP by 50%, hit all targets with almost any attack, and if you decide to use the Wizard Bracelet for the extra two slots, it'll boost your Luck and Speed by 50% (attack more often, and have a better chance for critical hits, or better chance of hitting with D. Blow).

---EDIT---
Just remembered, it doesn't give you access to Mug, but I never liked that command anyway (kept killing the enemies I wanted to steal from).