Most martial arts are pretty effective. The question really is: which system suits you?
What are your criteria?
Are you looking for kicking, punching, grappling?
Do you want to fight? Or do you want self defence?
Are you seeking a more philosophical component?
See what is on offer.
Does the class teach a 'complete martial art'?
Do you need to supplement the training with gym work, running or weight training?
How concerned is the class with health and wellbeing?
Are the students friendly and relaxed?
Is there a macho atmosphere?
Can smaller students use the art?