A general is not just a general, there is a huge difference in commanding a Division or the General Staff! Slim no doubt did a superb job commanding an Army but we have no idea how he would have performed at higher levels. Monty did a very good job at both Divisional, Army and Armygroup level. I know he usually wasn't the dashing type taking risks and winning stunning victories, but IMHO that was the last thing the allies needed. They needed first of all methodical types who at a low risk could apply the ever growing strength and not get out on a limp. He was just that and also a great leader of men. And even then he actually could take a risk, like at Markey Garden. The potential win was great but the gamble not too high. That it didn't work was mainly due to bad luck than bad leadership. Sure he was a bastard to be close to and a pompus arse - but nice guys rarely lead anything for very long.
My absolute favourite is Alanbrooke though. He did a good job as a Corps commander in France 1940 and after that as an able (re)organiser of British Home defence. His main contribution was as CIGS however - i.e. the main allied strategist in the decisive years of mid war. Not an easy job with Churchill as his superior but IMHO he is one of the most important persons in human history! If that can't bring him on top of British WWII generals - what can?
Slim did become CIGS in 1949. (After replacing his superior as CINC ALFSEA, and turning down offers from India and Pakistan to command their post independance armies)
And by all accounts did a fairly good job on handling the British and Commonwealth contribution to Korea.
In contrast to almost every other outstanding commander of the war, Slim was a disarmingly normal human being, possessed of notable self-knowledge. He was without pretension, devoted to his wife, Aileen, their family and the Indian Army. His calm, robust style of leadership and concern for the interests of his men won the admiration of all who served under him ... His blunt honesty, lack of bombast and unwillingness to play courtier did him few favours in the corridors of power. Only his soldiers never wavered in their devotion