I'll be honest, I probably don't know enough about enough of them to post anything sensible, but as already stated by Jon, I often regard the 'what ifs' of individual battles as being of relatively small impact in the grander scheme of things.
What if Harold's Saxons hadn't charged off their defensive position on Senlac Hill? Well, the Normans would still have been present in large numbers and would likely have retreated to fight another day, all the while bringing in more reinforcements from the continent.
What if Chelmsford hadn't divided his forces, leaving a shaky chain of command behind on the eve of Isandlwana? The defeat of the Zulus would have been brought forward a few months.
But if the Persians hadn't somehow lost their cavalry before the battle of Marathon (no-one really knows what happened to it - one likely answer is they were reimbarked on transports, or perhaps they were detached for scouting and foraging - but they seemed to have played no part) then the entirety of the world's history may have been changed, with Greek 'Western' civilisation being eradicated by Persian 'Eastern' influences, with the knock-on effect on both Europe and Asia.