The below table presents the list of all the available Focus Upgrades. It tells you which ones are the most useful.
Note! You do not need to restrict yourself to buying upgrade points from the Queen of Beggars only, because Garrett will be finding special symbols, while exploring the city and completing missions, which add points of this type without any additional charges. Unfortunately, such symbols are too scarce to obtain all the available Focus Upgrades, only thanks to them.
THE NAME OF THE UPGRADE
This is a two-level upgrade, where the first level should have already been unlocked, by default. Purchasing the second level grants you with finding additional clues (e.g. hand prints) which are helpful for finding collectibles. If you are going to use this guide to locate all of the unique loot, you can skip this upgrade.
This is a two-level upgrade. The first level makes it easier to pickpocket and is especially helpful if you do not want to knock down the opponents (or if you cannot do that, in the case of civilians). The second level allows you to pick locks (a modified minigame based on the lockpick use) and also is a useful upgrade.
This is a two-level upgrade. It allows you to discern the sounds in the environment. You can easily ignore them, because you can fare off by depending on careful and silent exploration of the area.
This is a two-level upgrade. It allows you to stun (level 1) or knock out (level 2) the opponents that you encounter, right after the direct combat begins. It is a useless upgrade, on condition that you are going to complete the game without being spotted (even if you are, you can try escaping, anyways).
This is a two-level upgrade. It allows you to slow down time, while fighting, shooting the bow or lockpicking. You can consider buying this one, in order to make lockpicking more effective but, this is not a priority.
This is a two-level upgrade, which decreases Focus consumption. Buy both levels of this upgrade if you use Focus often, especially that it runs low very fast, at the highest difficulty level.
This is a two-level upgrade, which makes you use the bow more effectively. The first level increases zoom while using the bow, where the second enhances the accuracy and the speed of using the bow. This is not a priority matter but, if you have too many focus points on you, you can consider buying this one.
This is a two-level upgrade, which allows you to stay in shadows more effectively (by muffling your steps over the regular and the noisy surfaces, "invisibility" while using up Focus). I recommend that you buy both pretty early into the game, especially if you are playing at the highest difficulty level.
The most useful Focus Upgrades
I recommend against spending your gold on new focus points, at the beginning of the game, at the Queen of Beggars'. They are not too expensive but, it is a way better idea to spend gold in the upgrades to your equipment and make sure that Garrett always has the right amount of the basic supplies on him.
From among all of the categories in the table above, I recommend that you turn your attention to three only. Dexterity helps you, to a considerable extent, stay hidden, while lock picking and pickpocketing of NPCs. Stealth decreases the risk of being spotted, while exploring the area and, you can especially appreciate it while walking over noisy surfaces (broken glass, water, etc.). Efficiency allows you to initiate the most useful functions of Focus, thanks to its decreased use-up.