Top 10 male and female lawn tennis players of all time 

1. Roger Federer - King of Grass, excellent on hard. Great serve, forehand, overhead, net game. Offensive & elegant game, unmatched consistency.
2. Rafael Nadal - King of Clay, excellent on other surfaces. Signature inside-outside forehand, great net game (great doubles player as well), heaviest topspin ever. Unmatched defensive game, mentally unbreakable, hardest worker on tour, nightmare to play against. Could have overtaken Federer if it wasn’t for his injuries.
3. Pete Sampras - King of Grass before Fed came along. Huge 1st and 2nd serve & forehand, including running forehand. Offensive game, flashy smashes, impossible to break.
4. Novak Djokovic - perfect overall game, great fitness and movement, great on defense, mentally tough in the second half of his career. Extremely impressive how he was able to improve his game and get out of the shadows of numer 1 & 2 ever.
5. Rod Laver - dominated Tennis before the open era, still managed to win Grand Slam in the open era.
6. Bjorn Borg - multi talented, dominated Rolland Garros & Wimbledon, also got to 4 US open finals. Hugely popular in the 70s, made tennis more popular globally. Perhaps retired to early to be higher on this list.
7. Andre Agassi - King of return, won on every surface. Signature fast paced game, known for hitting the ball early on the rise, taking time away from an opponent. Could be higher if not for personal issues, also one of the most successful players after 30th birthday.
8. Jimmy Connors - fan favorite, especially in New York where he won 5 times. Most singles titles ever (109), consistent over the years (second longest streak at number 1).
9. Ivan Lendl - huge resume with 8 major titles (and record 11 times runner up), 270 weeks as number 1, 94 singles titles. Known for his hard and heavy top spin forehand.
10. John McEnroe - intimidating, loud and controversial backed up the talk with dominant serve and volley game. His offensive style was perfect for fast courts of Wimbledon and US Open.

Edit: some comments suggested there should be women on the list as well, so here is my top 10 women tennis players:

1. Serena Williams - best serve ever, able to overpower everyone, also good movement. Dominated on all surfaces.
2. Steffi Graff - great forehand, movement and footwork. The only woman in the open era to win a Grand Slam, multiple years with 3 Slams.
3. Martina Navratilova - offensive serve and volley game was especially effective on faster surfaces. Also a great doubles player.
4. Chris Evert - best women clay player ever, also excellent on other surfaces, best winning percentage ever.
5. Margaret Court - dominated with her offensive style, big serve and volley. Much more athletic than her rivals at the time. Could be higher but a lot of her success came before the open era.
6. Monica Seles - most successful teenager ever, great on all surfaces except grass. Career greatly impacted by the stabbing incident.
7. Billie Jean King - won all grand slams and was also a great doubles player. Advocate for equal rights, part of her success was before the open era
8. Justine Henin - signature one handed backhand and a complete game. Success on all surfaces.
9. Venus Williams - great serve and power, especially successful at Wimbledon.
10. Martina Hingis - known for her finesse style, variety and movement. Great on all surfaces, also amazing doubles player.

Credits: Greg Jarucki