6505 or Triple Rectifier for Drop Ab Deathcore

the low end of an un-boosted amp sounds like shit to me now.

This, for anything other than cleans.

Seriously, you haven't played with a "tight" tone until you've played a boosted amp. Tubescreamers just give the tone so much more clarity and balls. More clean and focused, but more presence and liquid, gainy feeling at the same time... Even people using the Axe-FX amps and various free vst amp sims are boosting them with tubescreamer sims...

My favorite is the Maxon OD820, but the 808 and od9 are both good too... some of the ibanez are also fine.

EDIT: As far as settings, gain anywhere from off to 9:00, tone knob: 11:00 to 12:00, level at unity so that your guitar is at the same volume when the pedal is on or off... or maybe slightly louder if you want some extra push...