Although there are many options on how you can build your character in Witcher 3, there are several skills that really stand out from the rest. These skills are especially important if you're playing on Death March difficulty where every small advantage counts. Let's break down the must have skills:
Sun and Stars (1/1) - This skill allows you to slowly regenerate health during daytime, as well as giving you a small stamina regeneration bonus at night-time. Since you don't heal from meditating on Death March difficulty, this skill is very helpful to help you regen without having to use food or potions. I would recommend to pick this up as your first skill point.
Cat/Griffn/Bear School Techniques (1/1) - you should pick one of these skills if you're set on which type of armor (light, medium, or heavy) you're going to wear throughout the game.
Acquired Tolerance (1/3+) - This skill raises your maximum toxicity by the number of level 1 alchemy formulas you've learned. Considering you will be picking up lots of formulas throughout the game, putting even 1 point into this skill is very much worth it, so you will be able to use more potions before maxing out your toxicity bar.
Active Shield (3/3) - this is by far the most useful Sign upgrade, and should be picked by no matter what build you're going for. This gives your Quen sign an alternate version which allows you to hold a sustained shield that both blocks damage, and also heals you at the same time. This skill is absolutely invaluable; put 3/3 points into this.
Melt Armor (1/5+) - This is a great improvement to the basic Igni sign, allowing it to lower enemy armor. Even if you're using a melee build, you should still be using Igni in between your attacks from time to time. I like to put all 5 points into this since I need 6 points to reach Firestream and Active Shield anyways.
Firestream (1/3+) - This alternate version of Igni turns it into a continuous stream of fire, and it's ridiculously powerful. It not only has high damage, but can also damage multiple targets, and panic powerful enemies (like Fiends and Katakans) for its entire duration, along with removing heal effects. Putting 3/3 into this skill is ideal.
Puppet (1/3+) - This alternate version of Axii mind controls the target enemy and seems to work on most humanoid enemies. It not only automatically takes out one enemy out of a fight, but also gives you an ally that can really change the fight, especially if they have a special ability, like the Rotfiend's explosion. One point is good enough for this skill, although you can get the full 3.