What is the maximum wattage LED bulb that can be used in a 75 watt fixture?
The issue is heat.  The fixture is rated for a bulb using 75 watts and turning essentially all of it into heat.  

You could put a 75w heating element there and be within limits of the fixture.  

Seriously - you would not want to put a 75w (actual) LED there.  It would be like a Boeing 747 landing light.   Google some youtube videos of "100w LED flashlights" which people homebrew - they are portable suns.  

In any case, the fixture may not mind the heat, but LEDs and CFLs do not like heat and need to cool.   This is rather unlikely to be a problem for "75-watt-equivalent" LEDs of 7-12 watt range, or even a somewhat bigger 100w equivalent. 

But for our hypothetical 75-watt-actual mega-bright LED, cooling may emerge as a serious issue.  Or not.  There's a lot of thermal science involved which depends totally on the design of the particular fixture and the heat sink of the particular LED.  For instance a chandelier is not going to have the problems a weather-sealed fixture would.  But again we are talking about an extreme case of wanting vastly more light (lumens) than the fixture manufacturer ever imagined, and extreme projects take extreme risks.