The mods really depend on what you want form the site. These are my most used phpBB Mods:

SEO
shutbox
last visit
birthdays
Profile control
Attachment
quick reply
Cash
Topic calendar
Flashing GIF for new PM
PM Export
Main Admin Security
Anti Bot Question MOD
EasyMOD
Digests
Custom mass PM
Categories Hierarchy
Custom mass email
Yet Another Anti Robotic Register Flood
Stop Spambot Registration
Unique Registration Hash
Views
Save posts as drafts
points
Bank
SImple Merge
Subforums
subcathegories