For more details please see the Overview of Garlic page of this website. No, some are very mild and some are very strong and there is every taste level in between. For more details please see the Varieties page of this website and click on the name of any garlic and get a description and picture of it. As near as I can figure out, you lose very little in cooking it. The first thing you would lose would be the allicin and its antibiotic properties.
Heat breaks it down rather quickly, as do digestive chemicals.
Most of the health benefits from garlic come from the breakdown compounds, rather than allicin itself, anyway, as allicin breaks down in the body and it is the breakdown components that get into the bloodstream. They're the ones that really do most of the good. Even Kyolic Aged Garlic Extract, which has been soaked in alcohol and other chemicals for nearly two years still has anticoagulant effects as well as some other benefits, and it has no allicin left in it at all. Fresh garlic is better all the way around, cooked or raw. Cooked garlic would seem to have less immediate antibiotic value.