We received many well explained solutions to this problem. Martha from Baines Endowed C of E Primary School says:

Francis has the blackcurrantiest drink because he has two blackcurrants and one water.
Imran has the next blackcurrantiest drink because he has one blackcurrant and one water.
Ambreen has the second weakest blackcurrant because she has one blackcurrant and two waters.
Lizzie has the weakest blackcurrant because she has one blackcurrant and three waters.

You're right, Martha, well done, but I wonder if you could explain a bit more about how you compared the drinks? Boyang from Mountfields Lodge told us:

If Ambreen's drink has $2$ water and 1 blackcurrant, $\frac{1}{3}$ of her drink is blackcurrant.

If Francis' drink is $1$ water and $2$ blackcurrant, $\frac{2}{3}$ of his drink is blackcurrant.

If Imran's drink is $1$ water and $1$ blackcurrant, $\frac{1}{2}$ of his drink is blackcurrant.

If Lizzie's drink is $3$ water and $1$ blackcurrant, $\frac{1}{4}$ of her drink is blackcurrant.

Ambreen is A
Francis is F
Imran is I
Lizzie is L

I's drink is $\frac{6}{12}$ blackcurrant.

A's drink is $\frac{4}{12}$ blackcurrant.
F's drink is $\frac{8}{12}$ blackcurrant.
L's drink is $\frac{3}{12}$ blackcurrant.

F has the most blackcurrant so Francis' drink is the blackcurrantiest.

Pupils from Jebel Ali School, Dubai added to this by concluding that Francis had the blackcurrantiest drink, Imran was next, Ambreen's was after Imran and Lizzie had the least blackcurrant flavour.