Julia 1.0, 141(削除) 140 (削除ここまで) 133 bytes
~s=(d=Dict();split(replace(lowercase(s),r"[^\'r"[^\' 0-9a-z-]"=>""]"=>"")).|>w->d[w]=get(d,w,0)+1;(x,y)=findmax(d);count;sum(i->d[i]==x,keysx.==values(d))<2 ? y : """")
-7 bytes thanks to MarcMush: replace count(i->d[i]==x,keys(d)) with sum(x.==values(d))
Julia 1.0, 141 bytes
~s=(d=Dict();split(replace(lowercase(s),r"[^\' 0-9a-z-]"=>"")).|>w->d[w]=get(d,w,0)+1;(x,y)=findmax(d);count(i->d[i]==x,keys(d))<2 ? y : "")
Julia 1.0, (削除) 140 (削除ここまで) 133 bytes
~s=(d=Dict();split(replace(lowercase(s),r"[^\' 0-9a-z-]"=>"")).|>w->d[w]=get(d,w,0)+1;(x,y)=findmax(d);sum(x.==values(d))<2 ? y : "")
-7 bytes thanks to MarcMush: replace count(i->d[i]==x,keys(d)) with sum(x.==values(d))
Julia 1.0, 141 bytes
~s=(d=Dict();split(replace(lowercase(s),r"[^\' 0-9a-z-]"=>"")).|>w->d[w]=get(d,w,0)+1;(x,y)=findmax(d);count(i->d[i]==x,keys(d))<2 ? y : "")