Each gem is smooth and oblong, and about the length of a man's hand.
The seven gems, and a partial list of their owners are:
Time Gem (orange/red*) - Control over the flow of time.
Owners: Gardener, Thanos, Gamora
Space Gem (purple) - Control over space, allowing instantaneous travel across vast distances.
Owners: Runner, Thanos, Pip the Troll
Power Gem (red/pink*) - Increases physical strength and endurance.
Owners: Champion, Thanos, Drax the Destroyer
Soul Gem (green) - Control over the soul.
Owners: In-Betweener, Thanos, Adam Warlock
Mind Gem (blue) - Allows mind control and vastly enhances the holder's intellect.
Owners: Grandmaster, Thanos, Moondragon
Reality Gem (yellow) - Control over reality itself, creating and uncreating matter and energy.
Owners: Collector, Thanos
Ego Gem (gold) - contains the consciousness of Infiniti, when merged with the other gems reforms the cosmic entity the gems were once part of. Also grants the wielder the ability to manipulate dimensional energies. It was not one of the original six gems, but was formed when Loki attempted to steal the other gems from the existing holders.
Owners: Sersi, Loki
* Prior to The Infinity Gauntlet the Time Gem was orange and the Power Gem was Red, currently the Time Gem is red and the Power Gem is pink.
You can pick one and only one, after picking one the others are sealed away from you permenantly.
Gems I would eliminate right off the bat:
Ego Gem - pointless
Power Gem - nice gem, but not nearly as interesting as the other gems
Space Gem - see power gem
Gems I Would Consider:
Time Gem - time travel would be awesome
Soul Gem - sounds interesting, but do I really want to control souls
Mind Gem - mind control would be pretty cool
My Chosen Gem:
Reality Gem - my imagination would be my only limit to what I could do and this gem is exponentially better than the other gems, combined!
reality seems to be the general concensus, i have seen thanos use it to bring captain marvel back to life just to ask him a question...then unwished him soon after....thats hot!
Gender: Unspecified Location: Inside your computer
In a world with other super powered beings: reality or time.
In a world without super powered beings: power gem. I think it would feel good to be the physically strongest being on the planet. It would make me immortal in the eyes of a normal human.
Warlock transported himself into the Soul Gem, same trick he pulled on Nebula, but this time the Soul Gem betrayed him and helped Rune inside of Soul World.
i might be wrong but the only gem i see with the least drawbacks is the reality gem.
the space gem is traveling anywhere you want, might get boring after a while and all it means is if your being hunted down you can find a new hole to live in but thats just my opinion
the power gem needs its weilder to have a certain mastery over it doesnt it, i mean thor and drax had it but where still not able to over power thanos so im guessing they just didnt have what it took to unlock its full potential.
mind gem same as above also the fact that warlock messed with it so it cant be used to its full potential
time gem, hmmm i guess this would be cool to have i dont know about anyone using this item though.
the most fun one would be the reality gem i dont know if it was a canon comic but i saw thanos recreate captain marvel.
oh and i forgot the soul gem also has draw backs it will drain you too if you use it, sucky side effect