There are 3 groups that are particularly desirable
1st is the last of the 1300's - without the larger bumpers. They have all the benefits of the later developments (or most of them) but without the "ugly" looks (not everyone agrees about the ugly bit).
2nd is the last of the Fiat 1500's - again good developments and good anti-corrosion treatment so they last better
3rd is the last of the Bertone 1500's - best developments, especially electrically and anti-corrosion.
Early bertone 1500's are generally best avoided as they rust quickly - possibly the worst choice of any model.
The exact "best" years vary by region so you need to look at your local marketplace. In the UK it would be 77/78 for the 1300s, 81/82 for the Fiat 1500s and 86/87 for the Bertone's.
The federal market is a bit different with the extra engine choices and bumper variations.