Which is the best?

[peppy_man]

I'd like to know which form is the best among the following possibilities.

(1) in the last one and a half year

(2) in the last one and half year

(3) in the last one year and a half

(4) in the last one year and half

(5) in the last year and a half

(6) in the last year and half

I guess (1) is good, but not sure. Thank you in advance.

[Temico]

"During/within the last year and a half........."

[Tdol]

5 sounds best to my BrE ears. Also, I'd say 'years' in 1.