Sounds like what you want is going to be a Surefire X300. You can slide on/off anytime, just need to pull down on the tab and slide off to take it off.
Having said that, many will advise that you still need to identify your target prior to pointing a loaded gun at them so a dedicated weapon mounted light may not be favored by everyone. I for one use a dedicated light at home and still have my handheld light that I carry with me and also use.
Get both a handheld and weapon mounted light and train train train.
"But as for me and my household, we will serve the Lord." Joshua 24:15