What kind of knife should I use to slice tomatoes?
A really well-sharpened santoku-style chef's knife is usually my choice. But, it does depend on the type of slicing... such as thick/thin discs, wedges, or dicing.

While I understand that a serrated blade can help with slicing a super-ripe tomato, when I want really thin disc-shaped slices, something very sharp and preferably a thinner spine. The favorite in my kitchen is definitely a Henckel santoku chef blade. Some versions can have a very thick spine, and wouldn't work as well, though. This type of choice is also more utilitarian if you're cutting any other veggies at the same time, or making additional cuts in the tomatoes, such as when you're also going to be halving the discs or full-on dicing. 

Things I like the thinnest slices on are a ham and cheese melt, roast beef sammy, a sub for the road, and sometimes for a garnish. (hors d'œuvre, anyone?)

Also, when I'm cutting tomatoes into wedges, I also like to use this same type of sharp, non-serrated blade. Otherwise, I find it harder than necessary to do the smaller divisions after the 1/4-of-a-tomato size. I nearly always do 1/8 or 1/12 sized wedges for salad, so it isn't terribly often I use a "real" tomato knife. When I do, is when it's for a lunch and I don't want to sully a larger blade, or when doing large batches. Getting the core out is certainly easiest with a smaller, serrated blade.