I've owned my '71 Imperial for almost 18 years now, and it's still my favorite of the three years for exterior styling. Although the differences are slight (see http://www.imperialclub.com/Yr/1969/FuselageComparison/index.htm), they make quite a sizeable difference in the appearance of the car (at least to me, anyway).
On the '71, I especially like the headlight doors with the eagles, the IMPERIAL script spelled out across the front of the hood, and the
placement of the side moldings on the car. But the little eagles on the front fenders and hood of the '70 are nice too, as well as the IMPERIAL script spelled out on the rear bumper. And the grille treatment, cornering lights, and subtly different rear bumper on the '69 is nice as well.
I think the leather interior for '71 is rather plain, especially compared with '69 and '70. But I like the cloth interior for '71 better than either other year. Go figure.
I think '70 has the edge on interior trim with the embossed silver/black scrolling vinyl strip in the door panels -- http://www.imperialclub.com/Yr/1969/FuselageComparison/DSC00934.JPG
All three are nice, nice cars with plenty of good points to recommend them. While driving my '71 to the Anniston Meet this past weekend, I sure enjoyed looking in the rearview mirror -- seeing my '70 following along behind (thanks, Mark Jefferson!) was a real
treat!