The one before the prior one?

In normal understandable speech it goes like this:

• this year's model
• last year's model
• the model from the year before last

or

• this one
• the prior one
• the one before the prior one.

More formally the adjectives are:

• current or ultimate
• penultimate
• antepenultimate

but these latter adjectives are usually only used in very particular erudite circumstances, like where to put the stress on syllables in a word.

You can use current, penultimate, and antepenultimate, but I suspect most people won't know what those mean. More productive would be to simply say "this year's", "last year's", "2 years ago", "3 years ago", etc. Or, as Cameron says, call them by year: 2012, 2011, 2010, etc.

Prior suggests some sort of seniority to me, it is related to 'primo / first'. 'The bank has a prior charge over the property'.

'Previous' indicates time better, I think. You could try 'previous' and 'second previous'