MoonFlight
I own the whole first series but haven't read any more then the one series.
I'm planning to its just that I'm poor and once I start the second series I know I won't stop buying them since they are so addictive.
I lovE GreyStripe. He seems to be the best developed character of the series (as well as Bluestar). My favourite book has to be Forest of Secrets due to the whole Greystripe/Silverstream malarky.
I am in basically exactly the same position as you, only I've started the second series but I haven't finished yet, partly because I don't have the sixth book in the first series. I have one basic disagreement: on the most developed character, I'd have to say Yellowfang. However, if you're talking favorites, Graystripe is definetely in the running, but tied with Cinderpelt.