The majors are always about making history, and the four players who can make the most significant impact on the record book are Jordan Spieth, Rory McIlroy, Zach Johnson, and Phil Mickelson.
Spieth – If he wins his W-W-4t-W year in the majors would, IMO, be the best in the Modern Era (1958-now). Woods won 3 majors in 2000, but had no shot at the Grand Slam after the Masters (5th) and his last three majors were ball aided (he used the next generation ball while his competitors did not. Plus there was that bogus non-ruling on the final hole of the playoff with Bob May).
McIlroy - His fifth major in the last five years would keep him on the one a year pace he needs to match Nicklaus.
Mickelson – A sixth major would move him past Seve Ballesteros and into a tie with Nick Faldo and Lee Trevino.
Johnson – His third career major, his third different major, and his second in one season would make him a shoo-in for the Hall of Fame.