I will assume base install functions.Originally Posted by Dason
Man that's difficult. Like asking what body part do you need the most. How about if I give you a rank order with rationale)
1. dataframe (most used function hands down)
2. lapply (or apply family but usually I use lapply; second most used function)
3. do.call (awesome in so many contexts when you need to do the same function to an unknown number of lists)
4. %in% operator (I use this guy alot; see if each element of a belongs somewhere in b)
5. gsub (just a hard working regex)
6. strsplit (another hard working fast regex)
7. str (I like to see what's going on use this guy all the time)
8. head (great for looking at the data just to see if it's looking how you expect; another frequently used function)
9. : operator (very simple what to make integer sequences; very easy to use)
10. function (how cool is it that we can make any function we want with the function function)
Bonus: don't use it much but Reduce is awesome, as is parLapply for parallel computing.
I may start a top ten thread for R functions similar to this.