I have a 2-y/o (3 in Jan.) niece who would be happy with pretty much anything she gets (including the wrapping paper and empty boxes). She does enjoy the singing/dancing Elmo dolls (has Hokey-Pokey, E-L-M-O, one that says "Elmo loves you", and possibly another one right now), so she goes really nuts when the Elmo "Shout" commercials come on lately. Same with the "Tumble-Time" Tigger doll.
I also have a 4-y/o niece (5 in Nov.) who is usually happy with anything she gets also. She likes Barbie, and I was told she liked the Bratz commercial (but I don't think that's a suitable toy for a 4-5 y/o, due to the shoes and how hard the clothing can tend to be to get on the doll).
I babysit for a 9-y/o girl and a 6 (7 in Dec.)- y/o boy. The girl is into most things Bratz. She has pretty much all of the bedroom accessories and electronics on the market for them, a handful or two of dolls, some play accessories. From what I see, she doesn't really play with the dolls or accessories much, but she loves all of the bedroom accessories & electronics. She's also into Tamagochis, video games (has a GameCube and GBA SP), etc.
The boy tends to play with toys once or twice, and forget about them.
It doesn't seem like there is any one "got-to-have-it" toy for this year. Of course, we still have time.
For younger kids, anything will usually do.
For tweens/teens, I think a lot of them will be wanting a Nintendo DS, Sony PSP, Apple iPod, X-Box 360 also if it's supposed to be out for the holidays -- haven't paid much attention to that.