If Tiger gets to 19, and he likely will, he'll become the greatest golfer ever. Until then, I'm sticking with Jack.
One other point about Jack's 18 majors: he had to beat all manner of men who were hellbent on winning. Some of them, really, were a little crazy. These golfers won at least two majors in the Nicklaus years: Gary Player, Arnold Palmer, Billy Casper, Fuzzy Zoeller, Hubert Green, Raymond Floyd, Johnny Miller, Hale Irwin, Andy North, Tom Watson, Dave Stockton, Lee Trevino, David Graham, Julius Boros, Seve Ballesteros, Tony Jacklin. As a group, they beat Nicklaus more often than he beat them, and he still got to 18.
Here's the fivesome of golfers who have won multiple majors since Tiger won his first major in April 1997: Ernie Els, Vijay Singh, Phil Mickelson, Retief Goosen and Mark O'Meara. (Jose Maria Olazabal, Lee Janzen and Payne Stewart, all multiple major winners, each won one after April 1997.) The Els fivesome would have won in any era, including Jack's, but even combined they don't match Woods.
Nicklaus's wins in majors have the advantage of being much more memorable, and that plays into greatness. Think of Nicklaus going against Palmer, against Trevino, against Watson, against Father Time. What are the equivalents for Tiger? Beating Bob May at a PGA and Chris DiMarco at Augusta?
It's not Tiger's fault he doesn't have more competition. (Just the opposite.) If I had to guess, I'd think Tiger today and Jack in his long prime would have been very close in skill. Tiger is likely a better putter, chipper and bunker player. Jack drove it better, longer (allowing for equipment inflation) and straighter. As strategists, I imagine they'd be very similar. Tiger doesn't get nearly the credit he should for outthinking the field, and the course architects, too.
If Jack, circa 1972, played Tiger, circa 2007, in stroke play for four days a week every other week for a year, I bet it would be very close. If I had to lean one way, I'd go with Tiger, maybe 54-50, because I think he's more driven than Jack was, and Jack was plenty driven. Woods may well be the most skilled and dominant golfer ever, even if his competition hasn't been as stout. (Tiger's fields are deeper, but it's the talent at the top that matters.) He's done many things Jack never even thought about, most notably winning the Masters by 12 shots and the U.S. Open by 15. He's been on the national golf stage since the age of 15 but shows no signs of letting up, even as he starts a family of his own.
But for now, Tiger trails the Golden Bear by five. Big Jack's the leader in the clubhouse, and Tiger's just walking to the eighth tee. Time is on his side, but he still has a lot of work to do.