I've had chains break mid-ride and as long as I have a chain tool I can fix it and get home. With Shimano it doesn't work as well, because their pins are crap. With SRAM if you can balance it then you can get it back in and it works just as well as new.
For the longest time I carried a big, shop chain tool. It was my birthday last week, though, and I got a multi-tool with a chain tool on it....nice.
WAS carrying:
Tire levers, patch kit, two allen sets (metric and English), pump, chain tool
Now I carry:
Tire levers, patch kit, multitool, tube, pump, rain cover for seat.
I consider extra clothing as part of the day, not a took kit.
I like to have a phillipshead and a flathead available, but until I got my multi-tool I didn't typically have either. Having a tube, I found out, is a big priority because valves can pull out.
Good tire levers are key, because bad ones snap (heck, even good ones snap).
Spare quicklinks are handy, and I was carrying one and some extra linkage for a while, but I feel like chain emergencies are so few and far between, and I can fix them with just a chain tool as long as I have a pin that I'm not worried about it. Plus, I already used my emergency quicklink. I'll put another one in the kit next time I run by the LBS.