I'm not going to bother with a full review for this one because you just need to see it. That's all, just go rent it, buy it, watch it and be blown away.
I remember sitting through Hereafter in theatres and, while I didn't love the movie overall, I found the tsunami scene impressive at the time. After seeing The Impossible, the flood in the other film just looks like a giant CGI mess. The way in which they filmed the tsunami in this one (segmented scenes with real water) takes the realism to a whole other level that I haven't seen matched by any other movie yet.
The story, based on true events recorded by a family lucky enough to survive the Boxing Day disaster, is incredibly powerful and heartwrenching. I can't remember the last time I saw a film that felt so brutal and raw. It's horrible to watch but beautifully done, and the performances will blow you away.
As a side note, I also highly recommend the documentary Tsunami: Caught on Camera, which intercuts home video footage of the tsunami from several different countries with devastating interviews with some of the survivors from those locations. It's pretty hard to watch since, unlike the family here, most of them did not have a happy ending, but it is definitely worth seeing.