Strongest strain i ever smoked was a unknown strain from back in the day, early early 2000s. At the time i was a heavy smoker and I had moved away from my home town and came into some money so i returned to spend it lol. As soon as i got to town, I got two ounces off of one of my close friends and we rolled a joint and smoked it in his garage. This shit was so fucking good, it had me freaking out. I Got CONVINCED that he was going to rob me, which he would never do. But i had to get the fuck out of there when i couldn't feel my legs anymore. I then made up some bullshit excuse and stumbled into my truck and proceeded to get lost for over 45 min in my old neighborhood. I knew that place like the back of my hand but i had no idea where i was. Eventually i came down and over the next week, me and a bunch of old friends smoked the two ounces and every time after that, it had me laughing my ass off like i was a newbie smoker. I had been smoking for almost 10 years by that point, pretty much daily and was just starting into my grow obsession. It was amazing and it was NOT tainted with anything. I have done a lot of other types of substances so i am confident that it was clean. Clean ash, good taste, it was just straight fire. I WISH i had those genetics. Second place for strongest strain goes to one called "White Castle" from a med dispensary about 5 years ago in B.C Canada. Shit almost sent me to the hospital. It was just as strong as that unknown one, but it wasn't nearly as pleasant. Again, I was smoking heavily at the time, near full tolerance built up. Third place goes to a Mexican Sativa that smelled and tasted like fucking coconuts that i got on vacation, but that's another story.