If you are looking for a movement that is highly functional in its nature and can be loaded like no other, then the deadlift needs to be become and integral part of your training if it already hasn't.
The terms "Functional training" and "Functional strength" have, for some odd reasons, become synonymous with wiggling a 3kg dumbbell while standing on one leg on a bosu ball, doing maths calculations and singing happy birthday at the same time (I am joking, but I'm sure you get the point).
Picking up things off the floor is one of the most natural (and "functional") movements we can perform. Since the existence of time, people have relied on this movement day in day out. In sports-related areas, a strong deadlift gives better acceleration in the sprint, keeps a rigid torso during tackles, develops off-the-ground strength in grappling sports, and keeps a strong position in fast downhill skiing.
Most importantly, the deadlift is - when performed correctly - the king of exercises for a strong and healthy back until the last day of our lives.
It is a quite simple movement with two major technical considerations: back position and bar path.
The lower back needs to be extended (or slightly arched) throughout the entire upward movement so the bones of the spine create a lock-in mechanism together with the discs to bear loads safely. The entire spine and muscles surrouding it NEED to be compressed by forming an isometric contraction to create ridigness and a solid link between the bar and legs to lift the latter off the floor.
The trunk really is there for rigidness and compression. The deadlift itself is performed by the motor of the legs.
For lower back rigidness, think of squeezing your tummy between your knees towards the floor. This will create a greater arch in your lower back with simultaneous back extension - meaning the lower back muscles are now firmly contracted.
For thoracic spine rigidness, think of lifting your chest up while keeping the back angle the same. Do NOT make your back angle more vertical as this will lower your hips and decreases tension across the hamstrings and lower back.
In terms of the back angle, this will vary from person to person. A lifter with a longer torso and shorter legs will obtain a sligthly steeper back angle. A person with a shorter torso and longer legs will have a more horizontal back angle and relatively higher hips.
In any case, if the back angle is too horizontal, the quads are taken out. Since the quads play a crucial role in lifting the bar off the floor by a movement called knee extension, extending them too early in the movement will have you perform a stiff-leg deadlift, which is an assistance exercise for your hamstrings. Furthermore, if you lock your knees at lift-off, you create space between the bar and your shins, which creates unfavourable pressure on the lower back and increases the heaviness of the bar.
If your back angle is too vertical, your hips will lift too early to get the bar off the floor.
The movement of the hips should be that they extend the hip in the upper part of the pull, not to "stand up" to get the bar off the floor.
When analysing the bar path, it needs to be VERTICAL throughout the whole lift.
A vertically travelling bar means that the distance between the lift-off and lockout is shortened, making the movement perfectly strong and efficient.
Imagine, on the other hand, a bar travelling in an S-shaped curve. Not that this is something we would do even at the point of utmost boredom, it just illustrates how heavy the bar would feel when travelling this kind of path.
To keep a perfectly vertical path, the bar needs to stay in touch with your body throughout the whole lift. Any deviation of that will make the difference between a successful and failed lift, especially when operating with heavier weights.
If your knees lock out too early (as described above), the bar has to travel sligthly backwards to stay in touch with the shins, making it a non-vertical path and therefore wasting energy needlessly.
If your hips are too low, your torso too vertical, and your knees too far forward, you will probably have to roll the bar forward slightly to move it around the edges of your legs, making it an equally inefficient and heavier-than neccessary-movement.
When performed correctly, the deadlift is as functional as movement can get.
Plus - needless to say - you will increase the load on the bar for years to come.
Name another "functional exercise" that does the same.