Which army is the most flexible?

I was wondering, which warhammer 40k can most easily switch between different play-styles the most. Then i mean between being good at assault if i want to, then change some units, be good at close combat. Be good at annihilation, then suddenly being good at objectives. You know not always having to have one role.

Now i understand that there might be no army that can do this well or everyone would pick that army, but which army can benefit the most of being flexible.