I recently attended a comic convention here in Australia and it was a real blast, with terrific characters to photograph. I took a 70-200mm 2.8 and that was very useful in the subdued lighting, but the best lens would be a 35mm or 50mm fast prime. That will find the light for you and give you the sharp shots that are needed.

There is very little room at these sort of things and you rarely have more than 1-2 metres to back up from the people. Sure the 70-200mm is useful for the stage shots, but I found my best shots were from just stopping and asking other attendees if I could take their picture.
This girl was happy to pose with not much space.
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Are you looking at me? by Steve's Life, on Flickr[/img]
Same story here.
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Come with me, I know... by Steve's Life, on Flickr[/img]
Have fun, good luck with it.