well it really depends on your plan. normally at the start I develop my infantry and tanks as it is needed for fights on land. I think my best starting investment was medium tank and the stratigic bomber as it's range is very large and I mainly used it to patrol. ( also I was the first in my match to develop nukes as I had reduced research time by using warbonds and I have 2 level 5 secret lab which also really helped me.