Barra brings up Tiger Woods, and since I'm more knowledgeable about golf I'll use it as an example. But golf and tennis are very different, so someone else will have to take up that analysis.
The PGA Tour, as a whole, is far deeper today than at any time in the past. There are more players who can truly compete at the highest level now (just look at this year's major winners) than there were in the '60's and '70's, when Jack Nicklaus reigned supreme. However, the key is how many "great players" there are, as opposed to just very good. In that case, I'd say Nicklaus faced tougher competition, having to compete regularly against Palmer and Gary Player in his early days, and Tom Weiskopf and Tom Watson in his later years. They were truly exceptional talents. Tiger faces a very good crop of players, but none that has the same kind of intimidation and ability to win, as opposed to just play well, as a Tom Watson. The closest is Ernie Els, but even he's not at that level and there's noone else even close. So, great as Tiger is (and he may be more talented than Jack) he will have trouble ever measuring up if there's not a complementary group of rivals to test him.
I think Sampras probably suffers from the same lack of real competition. In fact, by these standards, McEnroe and Connors look even more impressive, since they had to face each other so often. But I don't know enough about the competition Laver faced to say how he compares. But I do know that, other than Agassi, Sampras never had to contend with many great players in their prime.