I'd say 5.50 Gen D-2.
D3 has a ****ing huge bug right now where a TON of games have corrupt game saves. Even better is the 'fix' for it just signs it with the broken code so the gamesave won't work on other PSPs anymore that have the PROPER hardware check.
Additionally, just know that moving to any Gen firmware means a total flash overwrite, so you'll need to re-pair the PSP with the PS3, resetup Flash and WMVs, re-add your theme and re-verify any PSN licenses.
However that means you miss the 'big' D3 feature which is the auto-patching of higher firmware files, but it doesn't matter since it doesn't handle them all, so you better get used to self-patching.
This whole scene has gone downhill in the last year.