This is somehow related to the mode question, where you can find many other solutions to get the most frequent level. I collected some one-liner solutions and also show solutions when there is more than one most frequent level.
#Create Dataset
x <- c("a","a","b","c","c")
#Some ways to get the FIRST most frequent level: "a"
names(which.max(table(x)))
names(sort(-table(x)))[1]
names(sort(-table(x))[1])
#Some ways to get ALL most frequent levels: "a" "c"
names(which(max(table(x))==table(x)))
names(table(x))[table(x)==max(table(x))]
names(table(x)[table(x)==max(table(x))])
#or the same but replace "table(x)" with "z"
z <- table(x)
names(which(max(z)==z))
names(z)[z==max(z)]
names(z[z==max(z)])