Sippy cup with a soft top and firm handles, but this was only because he was already on a sippy cup for his formula (I wasn't breastfeeding at that stage - about 9 months old for us). Then I moved to a hard-top sippy cup.
I agree with Giselle Maine - a sippy cup with a soft top is good because it at least slightly resembles the softness of a bottle nipple and it will feel somewhat instinctive for them to suck on it. And the sippy cups with harder handles are GREAT because usually when you're first introducing water, the baby is also in the throes of teething, and the hard handles are great for teething babies to gum on!
From the age of one I got my son a sturdy cup, it has a wide base to cut down on spillages and a narrower top to help his little mouth get round it to drink, some people prefer the sippy cups though, I give him those at night and get him the ones with very soft spouts and no valve.