Rosencrantz
Very well known Exeweb poster
You can take that two ways really can't you. Greatest material effect as in amount of runs and wickets, as described above, or biggest impact through Captaincy on results such as Mike Brierley, Stephen Fleming and, in the last year, Ben Stokes. There will be plenty of other candidates for both categories, I'm sure.Interesting question on the cricinfo site: "Which test captains had the greatest effect on their teams' fortunes ?"
On personal performance, maybe Gary Sobers, who was a bleddy nuisance in 1966 - or Allan Border, who propped up the Aussie team for years and got them winning.