The final Imperial fighter currently available (the Tie Bomber will be released later this year) is the Tie Advanced - the fighter which Darth Vader uses. The model itself is pretty cool but perhaps the main reason to buy it is to add additional flexibility and tactical options to your Imperial force. The Tie Advanced is so far the only Imperial ship with the ability to fire missiles and which comes with a shield generator.
Check my blog for more info about this fighter and the expansion pack contents:
http://anatolisgameroom.blogspot.se/2013/07/x-wing-tie-advanced-expansion-pack.html