She would do great. I am not saying that because I am such big fan of hers (and Federer), butas a player she was something else. The players who gave her the most trouble in her days were all players who had skills which aren't displayed in today's game.
Great serve and volleyer + lefthanded: Martina Navratilova
Power and those unbelievable angles, never-say-die-attitude + lefthanded: Monica Seles
Lefthanders gave Steffi trouble (see head to head), because of the different angles which made her hit more forehands from the forehand side (deucecourt) and most forehand winners came from the add court. (Check the us open match 98 against Schnyder)
In her own prime she was the one with the most power , the generation which started hunting her couldn't consistently beat her, and when she was the legend of tennis in her final years, the new crop (the ones with the power games + Hingis) could not really overpower her, because of her slice game, mental tenacity and forehand .
So many people, non-fans, regard her backhand as weak, but that shot was so perfected and good and difficult for an all power player to play with.
As other posters said. Serena Williams would be her most difficult opponent, Venus and Steffi would give the most entertaining matches. The rest of the field? Ivanovic and Jankovic would only win if Steffi has an off day, because champions like Graf (Seles, Evert, Navratilova) also knew how to win when their A game wasn't one. When Graf's A game is one, no one today would stand a chance.
Her serve is still better than many players (her hardest was 109 mph, with a racket from the 90s)
Her footwork (no one comes close, Federer maybe), but her drive to win matches and love for playing matches would make her a number one.
Andrea Jaeger said during the 1996 final (Graf - Seles) 1.04
http://nl.youtube.com/watch?v=79-UZKHHAIM&feature=channel_page
, Seles would much rather play the number 350 in the final (to get the match) and Steffi would rather play someone who is close to her (in rankings) to win the match. That show her passion and competitiveness which is unparalleled.