I like the Tommy Tippee cups - DS still drinks from those, ideal at night because they'd pretty hard to spill from (no valves). And they don't spill in my bag. He's going to school soon so I'll have to get him used to his big boy non-spill flasks now.

You can get open cups for babies - can't remember the name of them but they're sort of slanty so the child is more cautious about up-tipping them all over themselves.