I'm not a big fan of comparing players in different eras in any sport but; Serena is undoubtedly better.
Graf herself even admitted she thinks Serena will pass even Court's record of 24 Grand Slams.
The arbitrary number of Grand Slams doesn't make you a better player than the next, because the context has to be taken into consideration. People say Serena dominates women's tennis but Steffi stayed number 1 for over 370 weeks & the players she was playing against aren't near the level of female players Serena is currently up against. That's the definition of domination.
Most people who don't rate Serena usually do so because they may not like her as a person for whatever reason, but very few have any real reason to argue that she's not the greatest. Other than amount of Grand Slam wins, and i'm confident she'll beat that too.
The main attribute in which Graf > Serena is probably agility or amount of court she covers. But in almost every other attribute, serve, backhand, forehand, power etc. Serena beats her.
She's the greatest.