The trick to the ultimate hot fudge sundae is completely dependant on the quality of ingredients. For ice cream, I like Double Rainbow's vanilla, (by the way the fresher the ice cream the better the flavor) and for chocolate, there are many great brands out there, but basically the silkier the texture the better the fudge. The three little bars that come wrapped from trader joes are great. Personally im a milk chocolate fan, and normally use Cadbury's, but there's something about hot fudge sundaes that calls for dark.
I have found that simple is best.
For the fudge, i make a simple ganache with a one to one ratio.
1 cup cream
heat the cream until small bubbles form on the sides of the pot, (or microwave in jar) pour over a bowl filled with broken up bits of chocolate.
Wait a couple of minutes and then stir together til melted. its best served in large glasses (vs. bowls) pour some into the bottom of the glass, add ice cream (I do two scoops per person), and then pour more over the top, reserve some chocolate sauce and serve on the side, (round half way through chocolate lovers need a refill)