Well, we all know Mer and Der are in it together for the long haul. Just it in a nutshell.
Mark and Lexie most likely will get back together, but she'll probably get together with someone else before then, while he may not (I hope he doesn't).
Owen and Cristina are most likely IT too, I've got hope for them. Teddy shouldn't be coming between them anymore (and she better stay away from Mark too. Yuck. I wish she would just go away).
Callie and Arizona, probably.
These four main couples pretty much have good odds, MerDer without a doubt. Now, how about Alex, Miranda, as well as April, Jackson, and Teddy (yuck)? Alex will probably get together with April, Teddy, or someone new, though it's pretty hard to imagine him with anyone other than Izzie after all they went through together.