What is the most frequent word?

Jelly, 25 bytes

ṣ⁶f€ØB;”-¤Œl©Qµ®ċЀĠṪịµẋE

Try it online! or verify all test cases.

Pyth - 23 30 bytes

There has to be a better way to include digits and hyphens, but I just want to fix this right now.

Kc@s+++GUTd\-rzZ)I!tJ.M/KZ{KhJ

Test Suite.

Octave, 115 94 bytes

[a,b,c]=unique(regexp(lower(input('')),'[A-z]*','match'));[~,~,d]=mode(c); try disp(a{d{:}})

Accounts for the case with no most frequent word by using try. In this case it outputs nothing, and "takes a break" until you catch the exception.

Saved 21(!) bytes thanks to Luis Mendo's suggestion (using the third output from mode to get the most common word).

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_{The rules have changed quite a bit since I posted my original answer. I'll look into the regex later.}