I'd go for Slayer because they have different phases of their career, and they keep reinventing themselves, which is hard to do!
For me Divine was a low point for the band, and really the few albums that followed it were just.. bleh!! Diabolus, and that punk rock cover bullshit.
Early-early Slayer produced hands-down the most epic live material: Black Magic, Die By The Sword, At Dawn They Sleep are all killer live. As much as I like RIB I think it is a bit overrated, and the drums are pretty much kick-snare-kick-snare, pretty boring. High points for the album are of course Angel of Death (the most metal thing ever created, hands down) and the middle of the record, too: Altar of Sacrifice and Jesus Saves are fun. I am a bigger fan of South of Heaven as it was quite a departure for them musically, and easily the most diverse Slayer album. Are there some meh songs, sure, but I like the vibe and sound of that record a lot, and it has equally classic cuts as RIB: SOH, Mandatory Suicide. Even the oddball songs, like Dissident Aggressor, are fun to listen to anyway. That one reminds me of Eye of the Tiger, which makes me laugh. It's got a high school arena quality that is really funny to hear Tom sing. "STAND!" *weee-ooh* FIGHT!" *wee-oooh!*
Kind of stupid but I don't care. My high school mascot was the Tigers, so maybe that's why I actually listen to that track.
Seasons is the most commercially successful earlier Slayer, but it's also a bit more one-dimensional. That's probably what gets a lot of people into Slayer in the first place, but honestly it is just kind of the same deal the whole time.
What really did if for me with Slayer, though, was God Hates Us All and World Painted Blood, both of which I love! I was totally not expecting that shit when I first put in that GHUA cd, then I'm like, "Holy shit, it's actually good!!"
Seems the last handful of Slayer albums have been to a higher standard. Even Repentless is pretty good.
Nothing against Kreator, but I would argue Slayer has been influential of death metal, black metal, and they are pretty much recognized as the #1 REAL METAL band by everyone - even your f'ing Grandma knows Slayer is some badass, evil shit!!
Metallica is the band for wannabe posers. Slayer is the band for real metalheads!!