My personal favorite is the MAC short ram intake, if you don't live in an area of the country that is really hot. Short ram intakes offer good value because they generally cost less, and the MAC is priced right for a kit that is made specifically to fit the Focus with all the necessary hardware. The downside to the short ram intakes is the fact that the filter is on the inside of your engine bay, which means you are sucking in warm/hot air versus outside cooler air that a full length CAI kit will give you.
Other short ram intake kits on the market include K&N, FocusSport, AEM, OBX, and some other cheap-Os.
Full length CAI kits, like the AEM and Injen kits(AEM also makes a short ram), as mentioned have the filter outside the engine bay, therefore sucking in cooler air. Which is a good thing. The downside is the filter is exposed to the "elements", rain, snow, debree. I make a "weather sheild" with optional ram air snorkle for the AEM CAI kit, I'll let you look around the boards to find that information
The other alternatives are the "box" intakes. The Focus Central intake, the Volant intake, and the Iceman intake. The Iceman uses the stock box, replaces the paper filter inside the box with a performance filter, and replaces the upper and lower intake tubes with their tubes, and replaces the stock snorkle with a larger snorkle. The Focus Central intake is made like the Volant intake, replaces your stock tubing, box and filter. Each protect the filter from engine bay temperatures by enclosing the filter in a box. The Focus Central and Volant have the top of the box cutout for hoodscoop/ram air application.
There's plenty more information and opinions to be found on this board and other Focus boards. As the others have mentioned, it's been discussed over and over again, you won't have any trouble finding plenty of opinions.
No one mentions the K&N FIPK2 here. I have it and I like it. The engines bogs down when you punch it from a dead stop but thats to be expected from a short Ram. It has a heatshield and can be modified to accept an SVT snorkel. Only downside is it is kind of expensive and it is insanely LOUD.
k&n is decent for a ram but to be honest id rather have the f sport or mac.... f sports only 50 bucks cheaper than the volant tho which prob hurts its sales... if it was 175 they might sell more if ur trying to save money and just get a ram go with the cheapest and best, the mac