Really it's all about trial and error and what your needs are. Any of the apps in my signature are very good IMO if you want to try them out.... Some are pay and some are free....
a good one to get is the find my phone app. for people like me that tend to miss place there phone, it will over ride your settings and alert your phone so you can find it.
Oh yeah and KeepassBB v2. I store all my important info in there and it supports multiple platforms and storage in external files, unlike the BB Password Keeper.
I advise:
Quicklaunch
Opera Mini and Bolt Browser
Leave It On (I prefer this app to berry popup) - seriously, this is probably the most useful app you can get. Period.
Crunchsms or threadedsms (I prefer crunchsms)
if you sms a lot or like your sms's to look good
AP Mobile for news
I prefer BerryReader over Viigo for RSS, but I won berryreader - it's $10 and viigo is free, so... depends if you want (what I consider) the better app or the free one.
I use RIM's Twitter and (admittedly somewhat sucky) Facebook apps
Pandora for music, Nobex or Flycast or I <3 radio for AM radio
Dictionary.com
Mobipocket for ebooks
Google Maps
IM+lite if you want Facebook chat (I personally use nimbuzz because it has unlimited skype im)
BBM (you probably already have that)
If you play guitar, check out guitar studio by lca mobile.
Multi Click Icons is a nifty little app for keeping your home screen clean.
Finally, a weather app. I personally like bbweather a lot, but most people here on CrackBerry swear by berryweather (bbweather is free, berryweather is $10; that being said, if you have OS 5, berryweather is admittedly better. Once again, it comes down to quality vs price)
That's about all I can think of. Good Luck and welcome to CrackBerry!
Posted from my CrackBerry at wapforums.crackberry.com