Sam might have the most promising H2H but Serena has shown that she can give Sam a beatdown. She's done it twice actually. (Charleston 1&1, and Beijing 2&0) Serena has basically never given Li Na a beatdown, however she has given Na a bagel before, ironically in the only match where Li Na actually won. I guess you could say she gave Na a half-beatdown but that of course means zilch if Na ended up winning the match.
Li Na has the most overall competitive history (based on games won per match). She averages more games per match against Serena than any of them overall.
vs Serena : Overall Average-Games-Won-Per-Match
If you were to look at H2Hs merely since her comeback in June 2011, the figures are different... much to the displeasure of Maria who rivals only Errani
in terms of mugness against the entity Serena Jameka Williams.
Petra 11 2 meetings
Na 9 1 meeting
Caro 8 4 meetings
Sam 7 4 meetings
Aga 6.5 2 meetings
Angie 6.5 2 meetings
Vika 6.25 8 meetings
Maria 4.2 5 meetings
Sara 3 1 meeting
I added average games as a factor because even though Sam has the best H2H, she has demonstrated the mugility to succumb to Serena, where as overall Li Na will put up more of a fight. This question asked, which player gives Serena the most trouble? Even though a player has beaten you, you'd probably get more confidence going into a match against a player knowing you have pulverized them before rather than a player you never have (although in Serena's case, she has a bagel to seek comfort in).
Of course, these are just silly numbers which are probably skewed because certain players have faced her more times than others so don't take it too seriously.