The Guardians of the Galaxy trailer comes out tomorrow, and hopefully it will shed light on which gem the movie will focus on. Here is a quick rundown on which gems have already been covered, and which gems remain.
What We Know
Space- The Tessaract, despite its blue color, has been confirmed as the Space Gem, which grants its bearer teleportation and the ability to be everywhere at once.
Power- The Aether has been confirmed as the Power Gem, which enhances about every superhuman ability as well as exponentially increasing overall strength and durability.
Mind- It hasn't been confirmed, but many others and I believe that the glowing blue rock in Loki's staff is the Mind Gem, which gives unlimited psychic abilities.
What We Don't
Reality- Taken from The Collector by Thanos in exchange for one of the last of the Elder race; alters reality itself.
Soul- Taken from The In-Betweener; can rob an individual of their spirit.
Time- Taken from The Gardener; allows time-travel, alters the speed of time, allows prophetic visions.
What I Think
The Reality Gem seems the best bet for GotG seeing as it was originally held by The Collector, who is featured in the film. The Mind Gem would be my second choice, since the Soul Gem seems logically to be last seeing as it has numerous tie-ins with Adam Warlock.