I am of the opinion that thunderstorms are the most challenging weather condition to fly in regularly. Most other hazards have solid mitigation strategies or present such a high level of risk that the flight must be scrapped. Of course, this all vastly depends on your mission.
Moderate turbulence, for example, does not typically present hazards that would put your flight at undue risk, but I certainly would not take a first-time flyer in such conditions. Low visibility can cancel or change plans, and icing always takes careful consideration by planning your outs. High winds are usually accurately forecast, and only truly become an issue if a landing is forced beyond the ability of the aircraft or the pilot.
