it depends how old your dad is, or what period he prefers. my dad would like 50s/60s stuff better, the john wayne kinda stuff, like shane or true grit (which are both good movies), but i prefer the spaghetti westerns and clint eastwood stuf like...
good bad and ugly trilogy
a fistfull of dynamite
the wild bunch
once upon a time in the west
the magnificent seven
personally, if you like westerns you have to own those 7 movies. there are some good modern ones like unforgiven etc that others have mentioned. you can read up on the choices at imdb.com so you know what you are in for