I think its Seasons in the Abyss and its probably my favorite metal song of all time. The beauty of the song and with Slayer in general is their ability to create a spooky atmosphere with standard instruments and relatively simple songwriting. My other favorites are Spirit in Black, Jesus Saves, Mandatory Suicide, and Bitter Peace. But you can't really go wrong with the first four albums in general. I think once Kerry King started writing lyrics they got a little too gimmicky.
Angel of Death and the album Reign in Blood was integral to my music tastes growing up and really sparked a chain of events that shaped who I am. FWIW I think Raining Blood is the worst song on that album and kind of overrated in general, but maybe just because its played so often.
There's a lot of lesser known hardcore/metalcore bands out there, specifically from the 90s/00s who do a great job emulating Slayer influences into a fresh style. All Out War, Merauder, Ringworm, Integrity, Blood In/Blood Out, and Iron Age just to name a few.