This is the best knot to be used when attaching the leader to the fly, right?

If not, which knot is better?

its a real good knot for almost anything ! There are a number of applications for the uni . Its one of my main knots along with the palomer and the clinch to name a couple . I use a uni to uni for my leaders to running line .

I use the Uni knot when making my 30# spiderwire connection to anything and if done right, it doesn't need glue and rarely fails.

Plum,
Here's a good site with step by step instructions for knots:

http://www.marinews.com/fishing/fk_main.htm