Answer:
As a practitioner of Krav Maga I favor that style because of its practicality to everyday life and since it is quick to learn and easy to retain it has those advantages. People tend to favor what they know. You need to determine what you are looking for and hook up with a trainer with proven credentials and capabilities. Anybody looks like an expert if you don't know enough about the subject matter. Try www.kravmaga.com for information on Krav Maga and there are a number of excellent books on the subject. Haganah and Krav Maga have the same root in Israel from Imi Lichtenfeld in the 40's. Systema came from Russia and was used by the Spetsnaz. What they all have in common is they don't rely on fixed Katas as do other martial arts.