If you have never grew out a haze, I'd start off with early skunk haze or NHS. Then work my way to hazes such as critical haze or even mango haze. But as for the strongest goes I'd say NL5 haze and mango haze would be the two I'd go for. Some ppl who say SSH and others would say Afghan Haze. There's others who would even say nevil's haze. If u have time and room to grow out a 16 week plant like U2 or even the Stones..... Hazes aren't for everyone becuz of the time and care one takes and some are very picky about the conditions in which they are grown in.....but if u want the strongest keeper it would be mango haze.