Which type of dog should I get?

Looking for some insight to help me decide which type of dog I should get. Basics = I have 2 children 2 yrs and 4 months old, 1/4 acre back yard with a 6ft fence, semi active live style (I live across the st from a walking park and within a mile from a bike trail that I do take advantage of everyday except mondays I work on mondays) so the dog would get at between 45 mins and three hours of excercise everyday coming along with me but of course there will be times during the day when I will have to ignore it to take care of the kids. Im looking for a dog that #1 will be good with my kids, and #2 I want a dog that can be a protector kind of too. See I used to have an American Pitbull Terrier I got when I was 16 after being mugged by a man while out on a walk I didnt train it to be mean at all he was quite sweet I just thought that weirdos might think twice when I was with him. I wont get another young pitbull though bc of numerous family member concerns (whether warranted or not I care about my family and dont want them to worry) and bc there are just too many indesriminant breeders in my area i cant risk it with my kids and I dont want to risk furthering any bad tendencys in the breed. Im not getting another puppy from the pound I've gotten 2 and they have both died within the 1st week of getting them even though I let the pound know what was going on. I talked to a vet on how to disenfect my home and have fostered two other puppies between then and they did not get sick so my home is fine. So now that you've read my book : ) here's the just of my question: German Shepherd, Lab, Golden Retriever, or boxer??? If you have owned any of these tell me what you think. What you liked and didnt like etc.? THank you so much!