I think the records least likely to be broken are the ones that won't be able to be broken due to changes in the tennis schedule or general landscape of the sport.
For example, I think Rafa's Masters 1000 record (whatever it ends at) will not be broken because the levels of tennis tournaments change all the time. Masters tournaments didn't exist until 1990. I wouldn't be surprised to see them replaced with another type of tournament shortly after the current round of players retire.
Conversely, I think the record for grand slam singles titles (whether that ends up being with Federer or Nadal after they retire) is the most likely to be broken. Given how the surfaces are rapidly being homogenized and with all the talk of a 5th slam being added, it won't take much more than a player who's game is conducive to the homogenized surface to beat 17, 18, or 20ish majors.