Tam Tam didn't impress me when I was there.
Hobbyland Pochi was my fave, followed by Popondetta. It's very hard to find things at Models Imon because it's so tiny. What's great about Tam Tam is that it's a HUGE store where you can take your time browsing. They also had a lot of parts as well and a very complete selection track and accessories. In any case, I'd do all of them cause not every store has everything I want on my shopping list. Last time i was there, I actually ended up buying the bulk of my new trains from Tam Tam.
This one is off the beaten path:
http://maps.google.ca/maps?q=Akihabara+Station,+Japan&hl=en&ll=35.701405,139.771279&spn=0.000667,0.00134&sll=51.045,-114.057222&sspn=1.084444,2.743835&vpsrc=6&t=k&z=21&layer=c&cbll=35.701405,139.771279&panoid=IyXkEtBbvCQL-USmA1lGjQ&cbp=12,185.16,,0,5.05It's a store where people rent out pods and sell their wares. It's on the 2nd floor I think. There can be some very rare and unusual finds in here but do expect to pay a bit of a premium.