Of the three you selected, I'd say Raven Guard are probably your best bet. Space Wolves and Grey Knights each have their strengths, but they're really specialist armies and there are significant challenges to playing them well. If you go pure Grey Knights, you wind up with a low model count with very elite troops, which can be very problematic for a new player. Space Wolves, from what I've seen, are suffering from a lot of growing pains going from 5th to 6th edition and, while they can work, aren't quite the powerhouses they once were.
Raven Guard, meanwhile, have a brand new codex, a wide model range, and a lot of tactical flexibility. For a new player, I'd nudge you that direction. That said, I'll say what I usually say to new players trying to pick an army: pick the one whose models you think are the coolest. You're going to be spending a fair amount of money and time on these little things, and so loving the models makes the hobby a lot more fun.