My vote goes to 2011. Mainly because the shear awesomeness of Deconstruction.

Only Terria beats Deconstruction, in my opinion. My favourite Dev albums are probably:

1. Terria
2. Deconstruction
3. Ocean Machine

However, as many have stated before, the Dev album that appeals most to you depend highly on your feelings at that moment.

2011 was a year of maturity and realization. It took years to reach this point. Years of inspiration and learning.
1997 featured these foundations, which continue to stand the test of time. These later albums would be nothing if not for the emotion and necessity of City & OM. Therefore, '97 gets my vote.

Sorry, I mean, I think Ghost is one his best... But City was a masterpiece and easily my favorite album by him.