If you could have any Infinity Gem...

[Agent Orange]

Which would it be? Only one allowed.

Soul

This gem is sentient and has a hunger for souls. Allows the user to steal, control, manipulate and alter souls, living or dead. The gem is the gateway to an idyllic, pocket-universe.

Time

Allows the user total control over the past, present and future. Allows time travel, can age and deage beings and also be used as a weapon by trapping enemies or entire worlds in unending loops of time.

Space

Allows the user to exist in any location (or all locations), move any object anywhere throughout the universe and warp or rearrange space.

Mind

Allows the user to greatly strengthen and enhance mental and psionic power and access the thoughts and dreams of other beings.

Reality

Allows the user to fulfill wishes, even if the wish is in direct contradiction with scientific laws.

Power

Accesses all power and energy that ever has or will exist, and can back the other gems and boost their effects. Allows the user to duplicate almost any physical superhuman ability and become invincible.

I think I would personally go for Reality.