My single best changes were sorry I have 3 but if I had to chose one it was going dedicated. Couldn't have done 2 if not dedicated, well could but it would have cost an arm and a leg.
1- fixing all my urls to be more SE user friendly and doing a 301 from the old url to the new url.
2- Moving from a hosted account to a dedicated server were I had the control to do what I needed when I needed.
3- Took control of DNS from the hosting company to an outside source I have control.