completion-styles with increasing level of matching

[uzibalqa]

I am setting up `completion-styles'. Would like to have a system
where a user could switch values according to an increasing level
of matching. From least aggressive to most aggressive. What
would be the most useful transitions?

1. (setq completion-styles '(basic substring))

2. (setq completion-styles '(basic partial-completion))

3. (setq completion-styles '(flex))

4. (setq completion-styles '(initials))

Are `flex' and `initials' considered frequently useful? Which isthe most 
aggressive from the two?