Which forecast method is best to use and why?

Actual Window Fan Demand and Forecast
Month	Work Days	2011	2012	2013	2014	2015 Forecast
1	21	61,294	44,261	35,720	69,238
2	20	63,659	71,182	42,846	73,816
3	22	17,144	52,895	27,772	34,858
4	21	30,831	47,786	56,943	40,519
5	20	23,763	43,688	35,287	18,746
6	22	19,001	11,452	35,631	20,989
7	22	20,011	50,165	66,414	39,405
8	21	22,070	51,648	34,121	56,888
9	21	17,534	24,537	53,058	38,232
10	22	59,568	45,899	82,018	75,578
11	19	92,429	51,087	66,824	75,674
12	21	81,020	46,460	61,272	67,821
Totals	252	508,324	541,060	597,906	611,764
Monthly Avg.		42,360	45,088	49,826	50,980