I recommend the newest M33 for Slims.

I like the USB Connectivity feature:

'Upon connecting the USB, it turns into USB Mode automatically.'
For slims, combined with USB charge, it'd charge your PSP at the same time.

Amazing.

For Phats?
I suggest latest M33s, but people say OE-A.
It's an opinion.

But here's my note:
Use ONLY OE-A's/M33's.
DO NOT USE OTHER KINDS.
Example: Another provider that bases THEIR CFW off of M33's.

Please.
It may fuck you over on updates.