Besides a round brilliant, which cut sparkles the most?

[kittykat]

Which cut sparkles the least?

 

[DiamondExpert]

Probably an EC - but they have a quite elegant look if well cut.

 

[strmrdr]

There are other round cuts than RB that are likely the top sparkler option.


In a non-round probably the
jubilee
http://www.goodoldgold.com/jubilee.htm

or the square h&a cuts.
http://www.goodoldgold.com/square_h&a.htm