What date do i pick for end of year transactions, date check written (2015) or date funds deducted from bank (2016)?

The date you paid the bill - the date posted on the check, and entered into your ledger is the year the transaction took place. Typically it is the statement due date that would determine which month or year it is for. You should be closing the books each month, or quarter in a similar fashion, not just the end of the year. For payroll, the same thing - the final payday for 2015 marks the end of 2015 for reporting and everything. The first payday in January is in 2016, even though the time worked was in 2015.

2015

The date on the check determines the year that the expense was paid. The date that it clears is irrelevant.

When I lived in Missouri I always paid my property taxes on Dec 31. The checks never cleared until a week later.

And if the IRS asks you to prove you mailed the check by 12/31, how do you intend to prove that? If you are cutting it that close, pay for certified/return receipt (adds $7).

cash basis, the date is when it was written

it doesn't matter when the bank clears it