I am sure that Remi's recommendation is good, but there are several good options for slalom fins.
I have an '08 iS 111 and bought a Tectonics Tomcat 42 for it. I wanted one fin to cover my 6.6, 7.6 & 8.4 Maui Sails TR race sails. I find that the one fin works wonderfully well with all three sails, but I am not racing! Spin out is almost non-existent and speed seems excellent.
I weigh 77 kg so if you are going for one fin, the 42 may be a little large, especially if you will race downwind slalom.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.