-- Last Updated: Oct-15-13 12:41 AM EST --
If you can afford it, a Forstner bit is always better than a spade bit. If you happen to have both, it's a no-brainer, use the Forstner.
For plastic, a hole saw will also work well, and better than a spade bit. It depends on the diameter, of course, hole saws don't come less than 1-1/4", I think.
For diameters less than about 3/4", you can just use regular high-speed drill bits, be sure to use new, sharp bits and work your way up to the final diameter in 1/8" increments.
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