What's your budget? Shooting style? Type of work? Do you want to buy new or used? Do you have a large invest in lenses? Do you need to consider additional accessories (screens, audio, lighting, etc.)?
What camera should I buy is similar to asking, what car should I buy - it depends on what you want it to do and how much you're willing to spend to achieve that goal. All cameras have tradeoffs...there is no perfect camera.
For some jobs a t3i is fine. For others you'd want the full frame of the 5D2 or 5D3. For other jobs the 4K capable GH4 may fit better. And for other jobs you may want a Blackmagic Pocket/Cinema/Production camera. So what camera depends on you and what you do.
my budget is pretty low but around 2k i guess and lower. I wanna make short films shooting action sci fi fantasy adventure etc. i have a 17-55mm lens and 50mm lens both Canon. T3i is good but ive been looking into other camera that produce a better image like a fs100 or gh4 or 5dM2 or 3 so what should i get or what would be best of choice?