I look after events for IoD Berkshire and we spend a lot of time asking members what works for them. The best category is events that draw repeat attendees. A good example are certain types of breakfast clubs. The benefit comes from being able to build the relationship over time and staying in peoples mind by repeat contact. There is no specific format such as informal networking, speakers or topics that always works the best. However, the event has to provide the opportunity for attendees to easily get engaged in conversations with several different people. Of course the event has to be interesting to get people to attend in the first place.
My advice is to find an event that repeats several times a year that you feel comfortable at and keep going to it. Personally I think that weekly meetings only work for certain types of businesses. Going to the odd event and hoping to get lucky in finding a lead or good contact is increasingly unlikely.