Don't buy things that are easy to farm for! The following suggestions can only be bought:
1. Join Legion - 1200 ACs
Joining the Legion provides many benefits, with the most important one being new classes. If you haven't joined the Legion yet and want to know how, click here. It costs 1200 ACs to get in but if you complete it all within 24 hours, you can get back 1080ACs, making the overall cost just 120 ACs. Here are the classes that you could get by joining:
These classes can be obtained year-round with Legion Tokens (LTs). Includes: Infinite Legion Dark Caster, Yami no Ronin, and Legion Revenant.
Other classes can only be obtained during March-April for Dage's Birthday. Some cost LTs, like Legion DoomKnight and Legion SwordMaster Assassin. Others cost ACs, like Timeless Dark Caster and Immortal Dark Caster.
I suggest Infinite Legion Dark Caster (for farming) and Legion DoomKnight (for soloing) for new Legion members.
2. Nation Pets - 1500 or 5000 ACs
Nulgath's Birthday Gift - 1500 ACs
This pet has one quest that rewards many great reagents but it's only accessible by you. The downside is that it requires you to move around a lot. It's a lot cheaper than the next pet though.
Crag & Bamboozle - 5000 ACs
This pet is more expensive but also comes with a quest that can be farmed in one spot. It also makes farming Unidentified 13s a breeze. The downside is that its quests cannot be shared like Drudgen the Assistant.
(Bonus) Drudgen the Assistant - 5000 ACs
This pet is almost the same as Crag & Bamboozle, but synergizes very well with them to get the most used reagents for basically free. Crag's quest "Diamond Exchange" with Bamboozle's daily quest "GRUMBLE, GRUMBLE..." give free Unidentified 13s, which you can use in Drudgen's quest "Contract Exchange" to get Tainted Gems, Dark Crystal Shards, Diamonds of Nulgath, Gems of Nulgath, and Blood Gems of the Archfiend. However, this quest is only available to the owner of the pet, and it is up to you to decide whether that one quest is worth the price.
3. LightCaster - 1000 ACs
This class is not absolutely necessary to buy as it's restricted to level 80+ and there are better options at that point in the game. However, it's good for soloing and support and only costs 1000 ACs plus a bit of farming. If you want to get it before level 80, you'll only be able to use the downgraded version called LightMage, which is still a fine class - just make sure to use it with Mana Vamp.
4. ArchFiend - 2000 ACs
Due to this class' recent buff (15 Jan 2022), it has made purchasing other farming classes unnecessary. Instead, if you're looking to buy a great farming class right off the bat, I would go with ArchFiend, which can be bought from Oblivion at /tercessuinotlim. Keep in mind that this class can also be farmed at Level 50+.
5. Calendar Classes - 6000 ACs
ShadowWeaver of Time - 2017 Key 2.0
Most versatile. Does well at soloing, group DPS, ultra bosses, farming, and PvP.
Timeless Chronomancer - 2019 Key
Best soloing class available, also great in a group. Requires reading & practice.
Chronomancer Prime - 2011 Key
Simpler than Timeless Chrono but also less versatile. Good for soloing and group DPS.
Interstellar Knight - 2016 Key
Better at soloing than CMP. Use supports like Arachnomancer and Dragon of Time in groups.
6. Bank Pet - 2000 ACs
A bank pet allows you to quickly access your bank from anywhere, which can be very convenient. If you have one, all you need to do is press B to access it even if it's not equipped (it just has to be in your inventory).
If you want one and don't care how it looks, get either the Zorbak or Twilly pet from the Game Menu in Shops > AC Gear. If you want one that fits your aesthetic, wait for cosmetic seasonal events like TLAPD (November). Otherwise, you should claim a Bonus Package from within your online account manager and get either the Hollowborn ShadowRaptor, Necrotic Pirate Commander, Akiban Blademaster, or Apocalyptic Lich King packs as they all contain a bank pet.
7. Inventory Slots - 200 ACs
Purchasing these are useful because they are needed for so many farms, like Void Highlord, which require you to have a minimum spare inventory slot amount of about 15. It's also good if you like to bring a lot of cosmetics and classes with you so you can switch between them as you like. Free inventory slots are rewarded near the end of each year.