Who was better, Reggie Jackson or Dave Winfield? Why?

Jackson was better.

Let's begin with raw, basic statistics.

Jackson hit .262 in his career, with a .356 on-base percentage and a .490 slugging percentage. He did this over 11,418 plate appearances. Jackson hit 563 home runs, 463 doubles, 49 triples, and 1,509 singles, along with 1,375 walks, 2,597 strikeouts, 96 hit by pitches, 68 sacrifice flies, 13 sacrifice bunts, 183 double plays, and 123 reached on error. Jackson scored 1,551 runs and drove in 1,702 runs. He stole 228 bases and was caught stealing 115 times.

Winfield hit .283 in his career, with a .353 on-base percentage and a .475 slugging percentage. He did this over 12,358 plate appearances. Winfield hit 465 home runs, 540 doubles, 88 triples, and 2,017 singles, along with 1,216 walks, 1,686 strikeouts, 25 hit by pitches, 95 sacrifice flies, 19 sacrifice bunts, 319 double plays, and 150 reached on error. Winfield scored 1,669 runs and drove in 1,833 runs. He stole 223 bases and was caught stealing 96 times.

So, at first glance it appears that Winfield was the better hitter - he hit for a higher average, had a little less power, and got on base just as well. However, Winfield also played in a better era for hitters, having debuted six years later than Jackson (who debuted in the middle of a dead-ball era) - Jackson actually created more runs, relative to his environment. Jackson was also a better fielder - he was legitimately excellent when he was in Oakland, while Winfield was not good in New York (and did not deserve five of his Gold Gloves, while Jackson deserved between 3 and 5). Jackson also had better individual years - he was the best hitter in baseball for 4 or 5 seasons, while Winfield held that title once.

As an added bonus, Jackson performed better in the postseason, which I believe deserves a little additional credit (though I do not penalize players who perform poorly in the postseason or who never appear).