It's certainly not unanimous, but I think there's pretty much general agreement amongst current and former players and the general tennis fan population that Roger Federer is the best male tennis player ever, at least in the Open era. Borg has a better winning percentage and I think Sampras does as well, but it's hard to argue with 15 Grand Slam singles titles. It's a little fuzzier with the women. Justine Henin dominated the tour for a few years but lacks the longevity of a Serena Williams or Martina Navratalova. Steffi Graf also dominated for a number of years and has the only women's career Golden Slam (Grand Slam + Olympic Gold). People forget about Margaret Court, but the woman holds 24 Grand Slam singles titles which no one, male or female, has even come close to, but there are questions about the quality of competition in that period of women's tennis.
Of the people I have seen in my lifetime the two best - all time - are: Roger Federer and Serena Williams (Navratalova comes in a close 2nd)