Its kindof hard to say. There are a lot of shows that look very interesting. Its possible that Juuosei (sp?) could be the Spring season killer, but I only have the opinions of RAW watchers to go on.
The good:
Haruhi Suzumiya is off to a strong start. A very fresh approach to schoolkids anime and its not even properly started yet.
Ray the animation. Its too early to tell yet, but so far Ray is a good introduction to a good manga. The episodic nature of its source material, and its length, may mean that it may not be possible for the anime to fully deliver in the limited running schedule.
Ouran High School host club - quite familiar shoujo stuff, but the execution of it is pretty damn good. It also seems to be sneakily subverting the boys-love overtones. Special mention to the over all look and production.
Higurashi - cookie cutter schoolkids drama, with off the shelf character designs with a nice creepy horror undertone.
Himawari - surprisingly attractive production values in an underdog ninja show, that make me think that future episodes might be worth paying attention to. Too early to say where this one is goign though, could be action, harem, comedy etc.
Undecided:
Air Gear seems to go straight for the predictable low-budget Oh! Great manga adaption. I don't think its goign to be bad, but the format is watchable enough underdog/fights/fanservice (but with sk8ers).
Simoun made a mess of its first episode which was distinctly average at best, even if you approach it very apologetically. Weak dialog, disposable characters, and a low budget that doesn't merge the strengths of its mechanical design with interesting backdrops (totally clash) and the poor character design. Dialog is poor. Action atrocious in a series that needs it. I will give ep 2 shot to see if it recovers any. The first episode simply failed to introduce the series effectively.
The Third - too early to tell. Tomboyish fighting protagonist, dystopian desert setting, nice tank.
Magical pokan - this isn't going to win any converts. Pure ecchi fanservice, but its got an interesting look to it. Think of Popotan crossed with Bottle Fairies. Seeing as I liked those titles, and anything with Haruko Momoi (Komugi chan) as VA I like this, but I expect a lot of people to really hate it.
Gintama - well it was funny, but possibly too overkill on the parody and 4th wall jokes for my taste. It was also plain silly.
Haven't watched/reporting general consensus
Nana - what everyone is talking about. Very high potential. Very good and popular manga with a huge budget behind it.
xxxHolic - looks like a good anime adaption of a better manga than tsubasa.
Disgaea - won't win over new fans, purely for game-fans, may run out of energy quickly, animation quality very poor
Soul Link - by the numbers space-school anime done cheap.
High School Girls - this series underwhelms initially but improves as it develops
Zegapain by the numbers Sunrise mecha anime. Though thats not necessarily a bad thing if its all you want. {Production seems to be decent too.
Glass no Kantai Potentially decent but looks to be typically front-loaded GONZO animation quality and unlikely to be as good as its ancestors.