Unless you have any special requirements, I would definitely go with Postfix. Apart from being a very capable MTA in general, the part about it being the default choice also means that there will be more easily available documentation.
Assuming you answer properly on the questions asked during the install (apt-get install postfix) you will get an ok configuration to start of. Depending on whatever you also want to listen on SMTP you should choose either Internet with smarthost or Satellite system.
Also, see https://help.ubuntu.com/8.04/serverguide/C/postfix.html