Mulcair is more centrist for sure, he has already stated he won't raise taxes unless it's necessary.
Topp is a bit odd. Some consider him to be more left-wing and he has talked about raising corporate taxes by 50% and increasing taxes on high income earners, yet he tries to come off as someone who will run a fiscally responsible government and won't implement programs unless they can be afforded.
Nash is the biggest lefty in the group and more of a old time NDP, yet she'll allow provinces to implement private healthcare. She is more to the left then Layton.
Dewar is said to be a moderate. He probably wouldn't be much different then Layton.
Cullen I believe has described himself as pragmatic and seems to be more of a progressive rather than a "social democrat".