The best skills in Middle Earth: Shadow of Mordor are those that help you during combat. One of the most powerful skills is Shadow Strike which allows you to reach and execute lower enemies instantly. It is very useful if you are surrounded by orcs and need to get out, if you have a few pesty archers on top of towers or you want to kill a few orcs quickly:

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One of the best skills in Middle Earth: Shadow of War is Shadow Strike which lets you reach and kill weaker enemies instantly.

The best upgrade is the Chain of Shadows which allows you to chain together Shadow Strikes whilst you have enough Focus:

A powerful Shadow Strike upgrade is Chain of Shadows which allows you to chain together Shadow Strikes until you run out of Foxus.

A strong skill to go with that is the Rain of Arrows upgrade which restores Elf-shot when you perform perfect counters:

Rain of Arrows replenishes Elf-shot on perfect counters.

During combat you may get severely damaged. To replenish health you can drain orcs, but that can take a bit of time. The Consume skill lets you use might to quickly drain orcs during combat:

Consume lets you quickly drain an enemy which will replenish your health.

One of the most powerful skills is the Hammer of Eregion which freezes enemies when you vault over them. What makes it especially powerful is that it works on captains too which makes them a lot easier to kill:

Hammer of Eregion freezes enemies when you vault over them which is especially powerful as it works on enemy captains.