Anime Expo in California has the biggest dealer 
room I've seen, but I thought A-kon in Texas was 
more fun.  Haven't been to Anime Central yet.
Honestly though, if you're only going to a con 
for the cels, don't bother.  I really think there 
is usually a better selection online.  It has 
always been very hit or miss for me as far as 
cels at conventions, and if there are good cels 
they get picked up pretty quickly - the dealers 
room is a mob when the doors first open and there 
isn't much chance of even reaching a preferred 
dealer's table unless you wait in line for 
several hours before the doors open.  Once there 
there tend to be lots of poorly organized cel 
books to flip through to try and find what you 
want.
I don't think any con would pay for your room to 
have you as a guest for your car, but why not 
wait a bit on some of the detailing and use the 
money for a room instead? If you find people to 
share with it is a lot cheaper, and you can stay 
at a super 8 or something instead of the con 
hotel.