Which knots are best for ………

Trucker's hitch

http://www.netknots.com/rope_knots/truckers-hitch/

Don is my favorite. After that, I'll take a clove hitch around a trekking pole as a mid-point, bowline for non-adjustable loops, trucker's hitch (stupid easy) for adjustable guylines, and two half hitches for adjustable loops at stake ends.

I know how to tie a trucker's hitch which I've used in the past to fasten down things on a car/truck. That makes perfect sense to use to get the ridgeline piano-wire tight.

What knot do you use on the other end to fasten to the tree that will unfasten easily?

I'd suggest either installing LineLocs on your tarp for all the tie outs and the ridge lines (easiest, fastest adjustment)

or

Using the system demonstrating by Andrew Skurka here (lightest option, more finicky):
http://andrewskurka.com/2012/tarp-guyline-system/

I use a niteize figure nine on one end and a knot bone on the other. It adds a bit of weight, but it is extremely easy to use.

Double half hitch

http://www.netknots.com/rope_knots/half-hitch/

I like Skurka's version of the trucker's hitch, too. http://andrewskurka.com/2012/tarp-guyline-system/

Prussik slider with a small plastic hook attached. Throw a loop on the outer end of the line and you're good.

Siberian (Evenk) hitch for anchor

Truckers hitch for tension

A Four Wrap knot often works to tie and tension separate guylines on the ends instead of a ridgeline:
http://www.imrisk.com/hammock/speerknot.htm

+1 on AndyF's suggestion of the Siberian Hitch for a mooring knot that comes undone easily. It's one knot I don't seem to forget.

Here is a good demonstration: http://www.youtube.com/watch?v=RQ84BLdwLUQ

Farrimond Hitch reminds me a bit of the Siberian Hitch, both are easy and quick to do, and very quick release.