2nd is the only critical stage
It's probably fair enough to suggest that the second segment often, or even usually, is critical out there in flying land.
However, the takeoff climb limited part of the RTOW can be driven by any of the segments especially due to terrain.
Main thing for folk to keep in mind is that one needs to look at all the constituent limits when figuring the RTOW on the day.
While the rulebook is a bit quiet on how to get from the approach to the cruise it follows that one still needs to look at reconfiguration (third segment equivalent) and obstacle clearance. One is still just as dead after running into the hill regardless of whether the rulebook says this or that.