What is your favorite Plano box that can hold anything from spinnerbait and terminal tackle to hard baits? Looking for something around the 3700 size to hold a lot and I want this box to be versatile. Any recommendations?

How about a Plano 3750.  The front half of the box is about 4"x12" without dividers so you can fit larger things like spinnerbaits.  I believe this is the basic 3700 that most walmarts carry.  Thats where i buy mine.  I have about 6 of em with all different kinds of bait in them.

I'm a bank guy that lugs a backpack, so I actually stick with the smaller Plano boxes (3601). They fit plenty to meet my needs. Spinnerbaits have their own little pouch in the backpack as do buzzbaits. 

This...

 

Plano Double Cover Tackle Organizer https://www.amazon.com/dp/B001ECQPV6/ref=cm_sw_r_cp_apa_U8AYzbY97E1AN

 

The best part about it is the top cover interlocks with the bottom, so it keeps smaller things from migrating from one compartment to another. I used this box all the time when I was shore fishing.

I have been using 3600 size boxes and the 3650 style boxes are great for a universal storage box. So I would think the 3750 would be just as versatile but with more storage.

The new Planos have upgraded latches too, for around $3 at Walmart.