Elderberry jelly is the best. In the USA you can get it at a bizarrely named store chain called Christmas Tree Shops.
They did Elderflower jam here, but I didn't buy it as I would've eaten it straight out of the jar.
I think rose petal jam is the best, it tastes amazing and I love the texture of the rose petals.
Those wild hibiscus flowers in syrup taste amazing too, though it's better when it's put into a drink as otherwise the flavour is too concentrated for me. I love the texture of the flowers too.