Welcome to the forums!
Best apps is a very wide ranging question... it depends what you want to do with your phone... do you want to use if as a social media device, to play music/videos, do a lot of web browsing... the list could go on.
Here are a couple of apps that either I have found useful or others have suggested to me:
Barcode Scanner - does what it says on the tin!
Bump - used to transfer files from one phone to another (both phones must have the app on)
Double Twist - music player (and
Air Sync - lets you transfer stuff between your pc/mac and also allows PS3/Xbox streaming)
Dropbox - 2Gb of free online storage that can be accessed from your phone - follow my link and get we both get an extra 250Mb -
http://db.tt/g9awoEX
Lookout Mobile - like secturity for your mobile... it checks permissions of apps and lets you know they're ok, as well as having a few other cool features.
Google+ - 'cos it's great!
MyPhoneExplorer - you need the app and the PC/Mac version... it is a sync app... better than the HTC one that came with my phone
Dolphin Browser HD /
Dolphin Browser Mini - Internet browser (better than stock)
Opera Mini - another internet browser (again, better than stock)
EDIT - most of the apps I have mentioned are free - there is at least one (Air Sync) that is not.