Jennifer no doubt - with her back up against the wall against Serena - she faught like a lion, and gave Serena the most intense rivalry she has experience, cant say that about Maria.
Plus, Jennifers first period in the top ten was 1990-1993, probably the most difficult time ever to win your firtst slam or break the top 5. Her peak was 2001-2002, and 2003-2004 to a lesser extent, when the competition in womens tennis was at its highest level in the top ten IMO, yet she was ranked #1, won 3 slams, would have won more in any other period.
If were talking Jennifer 2001 vs Sharapova 2006 in a grandslam final - Jennifer would take 3 of the 4 (Sharapova might edge Wimby, but who knows!).
Serve - Maria, altho its becoming more and more of a liability
Forehand - Jennifer ALL the way
Backhand - Maria
Movement - Jennifer
Stamina/ability to rally - Jennifer
Mental Strength - kinda even, when deep in a big match dogfight, Jennifer (go watch AO 02 final, or RG 01 final!)
Volleys/touch - Jennifer