Check out @peegore reviews of oneshot concentrates, a good read and very informative.
For me the best I've had in order are:
Uni Milk - Chefs flavour (Cuttwood Unicorn Milk but better IMO)
Athena - Dark Star (Strawberry, Kiwi Bubblegum)
Pineapple Blitz - Drip Hacks (Tropical Lucozade)...