The ThruNite TN11S can go from off to strobe instantly, I don't know if it can go to strobe instantly while on, or if it has to cycle through other modes.
Fenix TK75 goes to strobe in any mode or even when off by holding the mode button for aprox 2 seconds. Push mode button again and returns to last used level. SoS works the same
way, except need to push mode button for aprox 3 seconds and goes thru strobe to get to
SOS.
Klarus xt11 or RS11. I have the RS11 and the two body switches (raised button is on-off with momentary and the lower switch is strobe only) are very easy to access. The xt11's second tail switch is a mode switch that strobes when you hold it down. Both lights are very good but I prefer body switches. The xt11 is a tactical light versus the rs11 is more intended as general purpose.