Nasturtiums are definitely the tastiest. You can eat the seeds heads as well.
You can also eat the common lawn daisy though it doesn't taste of much. Chive flowers taste of onion, sweet violet taste sweet, borage flowers taste of cucumber and marigolds are a little spicy.
Rose petals are edible and slightly sweet. Look beautiful layered with matching coloured fruit. Some have a hint of spice to them.
Most vegetable flowers are edible but not all.
Eat your weeds too. Dandelion flowers are sweet if young, clover is also sweet. Queen Annes lace, the original carrot. Tastes like carrot! Just be sure you have the right plant as some very poisonous plants look very similar.
Peony flowers are edible although I have never eaten them as I prefer to look at them.
Last but not least, apple blossom but not too many as, like the pips, they contain cyanide.
Check this site for some mouth watering recipes.
EDIT...forgot to say I have just finished making my second batch of Elder flower juice and the next picking will be made into fritters.