Settings that I found most nice to play with:

For fast weapon switching:
hud_fastswitch 1
sv_defaultdeployspeed 4

run the command: "togglesprint" (from an addon)

Sliding ability:
sliding_ability_acceleration 100
sliding_ability_acceleration 0
sliding_ability_cooldown_jump 0
sliding_ability_max_speed 800

ULTRAKILL Dash:
ultrakill_dash_mul 1.5
ultrakill_double_input 0

ULTRAKILL Enemies: 4x Player Damage, enable healing from blood for all enemies, Difficulty: Violent

Armor on Kill:
Only headshots give armor?: Enable
Count one-shots as headshots?: Enable
Armor per kill: 1.5 (15 armor)

Holstered Reload:
4 seconds
Regen all ammotypes available

Global NPC Spawner:
Antlions (1/3 chance): Antlion (1/10 chance) & Antlion Guard (1/35 chance)
Bosses (1/100 chance): Cerberus (1/6 chance), Mindflayer (1/3 chance), Minos Prime (1/10 chance), SwordsMachine (1/6 chance)
Combine (1/4 chance): City Scanner (1/15), Combine Elite (1/20), Combine Gunship (1/100), Combine Soldier (1/10), Manhack (1/15), Metro Police (1/5) forced stunstick, Prison Guard (1/15), Prison Shotgun Guard (1/15) forced pistol, Shotgun Guard (1/30), Strider (1/100)
Zombies (1/2 chance): Headcrab (1/10), Zombie Citizen (1/5), Zombie Medic Torso (1/8)
Spawn distance min: 1000
Spawn distance max: 5000