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The phazon suit.

the phazon suit is the best and its black

I've never seen the light suit, so I'm going with phazon.

so far I'd say I like the light suit the best as it's less bulky than the rest.
but then again there is nothing better than watching Samus burst in with the Varia suit and kick arse.

but why hasn't anyone mentioned the PED suit from Metroid prime corruption?

My vote on this poll goes to the VASTLY under-voted Fusion Suit as it looks the best when it's fused with the SA-X's Powersuit, more streamlined and less bulky than samus' suit with the tri fold advantage of triple layer defense reduction.

Here's how the different suits factor out:
Zero Suit Base of all suits, has no damage reduction
Power Suit Devoid of Varia Suit. Little damage reduction, stacks over Zero and Fusion Suits
Varia Suit Stacks over Power and Fusion Suits, Adds extreme temp protection, Damage Reduction Rating 1
Gravity Suit Stacks over Varia Suit, Allows for movement in water and lava safely, negates acid damage, Damage Reduction Rating 2
Phazon Suit Stacks over Gravity Suit, Allows safe contact with blue phazon and use of Phazon beam, Damage Reduction Rating 3
Dark Suit Stacks over Varia Suit, Gives partial immunity to Aether's Atmosphere, Damage Reduction Rating 2
Light Suit Stacks over Dark Suit, Gives full immunity to Aether's Atmosphere, Damage Reduction Rating 3
Fusion Suit Base suit, is a surgical remnant of the Power Suit that was mutated by the Metroid DNA used to cure Samus of the X parasites. Damage Reduction Rating 0
PED Suit Stacks over Varia Suit, Allows use of Hypermode charged by Phazon, Damage Reduction Rating 2
Hazard Shield Stacks on PED Suit, prevents damage from Acid Rain and from Fuel Gel contact, Damage Reduction Rating 3

Although Hazard Shield isn't an actual Suit, I included it so that it is understood what it does and how it relates to the other suits. Following is the stacking orders available for use:

1. Zero > Power > Varia > Gravity
2. Zero > Power > Varia > Gravity > Phazon
3. Zero > Power > Varia > Dark > Light
4. Zero > Fusion > Varia > Gravity > SA-X Power
5. Zero > Power > Varia > PED > Hazard Shield

Stack 1 is for Metroid Zero Mission and Super Metroid. Stack 2 is for Metroid Prime. Stack 3 is for Metroid Prime 2 Echoes. Stack 4 is for Metroid Fusion. Stack 5 is for Metroid Prime 3 Corruption.

As you can see, all the suits are interlinked by being compatible with Zero and Varia layers. well, I think I've rambled on enough... once again, PED + Hazard shield = win!

True that. Plus the light suit prevents damage from Dark Aether and it's waters. Combine that with the light fields that heal you, and you pwn.