Use it to invest in RRSPs, and use the tax savings to
contribute to your RRSP.
If you earn $60,000 per year and contribute $6,000 (10%
of your earnings), if your marginal tax rate is 30% you will get $1,800 in
tax savings. When you contribute the $1,800 to your RRSP it will generate
another $540 of tax savings. When you contribute the $540, it will generate
another $162 of tax savings, etc., etc......
In order to have the same after-tax money as in #1 and #2
above, you will have to contribute about 15% of your earnings to your
RRSP. You can then do what you want with any tax refund.
If your contributions are made through payroll
deductions, you may be able to reduce your taxes right away, instead of
waiting until next year to get a refund.
With this option you will probably have the most money when you
retire, but market volatility may keep you awake at night. If it does,
pay down your debt first.