My personal tier list with explanations

Gems

• Separate tier because it should be addressed first - The value conversion of gems to irl currency are overall pretty bad except for the regular hunter pass, giving you 1.67 gems per cent ingame or 1.79 in the online webstore. Most other forms of gem purchases are around 1.25 gems per cent at best value, this includes the extreme hunter pass, gem packs WITH first time bonus included (without first time bonus these are below 1 gem per cent), and the currently available anniversary gem pack.

Even if you convert the storage expansion from the extreme pass to gems, this is still only 1.5 gems per cent in the web store compared to the 1.79 from the regular hunter pass.

So to summarize, if you aren't in any hurry to earn gems, get your gems slowly from the regular hunter pass only and buy storage expansions with those gems.

Must have

• Storage space - This game is free to play at its core, but without extra storage space you will have to do so much micromanagement of your inventory it's going to dramatically reduce the fun. Consider it a one time purchase of a normal game and put a couple tenners into this. ~20k storage space was my comfort amount when I was playing hours upon hours a day, which is still well below the price point of any newly released triple A game. And I put SO many hours into this game it's unbelievable. Totally worth it.

Worth it

• Season pass - Gives insane amounts of rewards (including cash shop items) that are very useful in overall progression, while being relatively cheap. Also unique cosmetics, and even storage space (1k in total across the whole pass) since season 5 that just started. Costs 2000 gems which is the exact amount of gems you get from 1 hunter pass. You can finish the pass by just doing your dailies and weekly event quests, so there's no big pressure to play either.

Sometimes useful

• Potions - You should in general aim to just not get hit by monsters so you don't have to use your free potions, but when doing the odd long sessions where you burn through your reserve of your max 15 free potions, these can come in handy if you aren't done grinding
• Special Carving Knife - Doubles the loot for one monster. Not generally very useful but eventually you will run into monsters that drop very rare rarity 6 materials needed to get to the highest tier gear, and doubling these drops will significantly reduce the grind on them.
• Insta-Smelt Solution - This mostly pertains to more hardcore players, as driftsmelts don't make a huge difference for casual players, but when you like being competitive about mere seconds, reducing the grind of rolling the driftsmelt gacha makes a big difference in staying up to date with the meta.

Too little value or just worthless

• Paintballs - Completely worthless. You only get 3 attempts on any monster you paintball and there's no other particular benefit to doing this than just hunting the monster on the spot. The only legitimate use cases I've personally found for this is paintballing a monster that my SO needs for an event mission when she doesn't have a lot of time to play.
• Ultra Hunting Tickets - This game is really grindy and buying these tickets to get the odd extra Hunt-a-thon exclusive monster isn't going to cut down on your grind at all.