As far as what is THE BEST assistance work for each lift day. Any thoughts?

squats

The BEST assistance work is.....what works for you. My assistance work I do might not work for you or anybody else. Just like in the book, Don't sweat the assistance work. Worry about the main lifts.

Welcome to 5/3/1!

Things will vary for each person. What works for me might not work for you, we're all going to have lagging muscles that need to be braught up.
I spent a while doing the Cube Method, and I saw a lot of similarities to 5/3/1. I've now gone back to 5/3/1 and taken a few concepts from the Cube. Mainly making the 4th workout (overhead) a bodybuilding type day where I'll hammer some weak points, do some curls, etc

What I've narrowed it down to is this (bare bones, I'll add some extras in on days I have time):

Monday:
Deads (5/3/1) percentages. Then I do 1 set of deficit pulls around 60-70 percent for reps just don't go all out.
Squat variant.5-12 reps per set I use my safety squat bar, or you can do a standard back squat, or front squat. I like to work in higher rep ranges.
Abs
Prowler

Wed:
Bench (5/3/1) superset with 5 pullups.
DB Press. Flat/incline/decline for higher reps. 12-20 reps per set.
Back work. (some type of row)

Friday:
Squats (5/3/1) I also do a dropset after my heaviest work set. And I'll hit somewhere around 60-70% for some 3 second paused reps. I've gotten extremely comfortable with the bottom position of the squat because of this.
Snatch Grip Deads. I vary the reps and percentages but I typically do 10-20 reps per set. These will fry your hamstrings and glutes.
Sometime's I'll do SLDLs instead of the snatch grip deads.
Abs
Prowler

Saturday (I make this closer to a bodybuilding day):
Overhead Press (5/3/1) superset with 5 chins
BBB Bench (50%x10 for 5 sets) superset with barbell rows (if you're doing heavy rows you may want to just do them after)
Pushdowns or a triceps movement
Curls
Whatever Else


I agree with Foxy, find the movements that make the biggest impact on your lifts and stick with them. Don't major in the minors or think too much about assistance. If you hit the main movement, and one good supplementary lift, you don't have to worry about much else. I'll push up the volume if I have time to blow, but I've had some great workouts where I pulled deads, hit a few sets of squats and walked out of the gym.