My personal favorite Metallica song is Until It Sleeps just because I like the catchy repetitive riff. It also shows a softer side to Metallica but still manages to sound 'heavy'. However, Until It Sleeps is not the best Metallica song overall. That would be Master Of Puppets not only because it is a classic but because it shows Hammet's talent adequately.
Enter Sandman IS good, but it's no match for Sad But True (Hey I'm your life I'm the one who takes you there hey I'm your life I'm the one who cares they they betray I'm your only true friend now they they'll betray I'm forever there
I'm your dream, make you real I'm your eyes when you must steal I'm your pain when you can't feel sad but true
I'm your dream, mind astray I'm your eyes while you're away I'm your pain while you repay you know it's sad but true
you you're my mask you're my cover, my shelter you you're my mask you're the one who's blamed do do my work do my dirty work, scapegoat do do my deeds for you're the one who's shamed
I'm your dream, make you real I'm your eyes when you must steal I'm your pain when you can't feel sad but true
I'm your dream, mind astray I'm your eyes while you're away I'm your pain while you repay you know it's sad but true
hate I'm your hate I'm your hate when you want love pay pay the price pay, for nothing's fair
hey I'm your life I'm the one who took you here hey I'm your life and I no longer care
I'm your dream, make you real I'm your eyes when you must steal I'm your pain when you can't feel sad but true
I'm your truth, telling lies I'm your reasoned alibis I'm inside open your eyes I'm you
sad but true)
And my 2nd favorite song would have to be Master Of Puppets (And then Enter Sandman!)