Ilevel on FFXI does not work as it does on every other mmorpg.
Ilevel on every other mmorpg is very simple, there is one set at each Ilevel (oustside of pve/pvp) and that is that. You get the highest set, done till next patch.
On XI, Ilevel started out this way but players hated it and so they stopped at 119. Now 119 is a base level and in that Ilevel there are vast arrays of gear and the quality of them differs greatly.
The main function of Ilevel now is trust level (trust level is based off average Ilevel of weapon,head,body,hands,legs,feet, that you can find by typing /checkparam <me>, aggro, check status of monsters, XP rates in escha is impacted by this.
Also for BST (main weapon Ilevel levels your pet), for avatar it's the ammo slot with a sachet, and for pup it's the frame ilevel).
It's also worth noting that Ilevel is calculated in real time, so you should never macro in any gear under 119 unless you want your trusts to level down and possibly get one shot (you can see this as their hp will change as you change ilevel of gear)
Gear to aim for if new is escha drops (can also get from domain invasion (400 or 800 point armors)) and/or ambuscade that starts at base and can be built into +1 or +2 gear. Also some artifact/relic/emp gear fully upgrades to 119++ depending on stats and job.
https://www.bg-wiki.com/ffxi/Zurim
the way 400 point stuff works is you get the Abjuration
, for instance for thief you would get Jovian parts, then buy a vexed or jinxed (hq) cursed part from the auction house and then take both items to an npc in north san'doria to create the final armor.
see Purifying cursed items on
https://www.bg-wiki.com/ffxi/Category:Abjurations
you can then add augments to this gear you just made:
https://www.bg-wiki.com/ffxi/Nolan
For instance if you got Adhemar / jovian, there are 4 paths you can pick from and upgrade, path A is the norm for thief TP sets.
the way 800 point armor works is,
Keep in mind you can have multiple duplicated of the 800 armors, with lots of different augmented stats on them. so 2-3 different herculean bodies for instance.
https://www.bg-wiki.com/ffxi/Category:Ambuscade
Gear above this would be Odyssey sets or Lillith (malignant gear).
A common rule of thumb is gear with good stats for your set = good, gear with good stat for your set + -dmg taken by you = much better. People will even take worse gear with -dmg within reason.
Ody
https://www.bg-wiki.com/ffxi/Ikenga%27s_Attire_Set
https://www.bg-wiki.com/ffxi/Gleti%27s_Armor_Set
https://www.bg-wiki.com/ffxi/Sakpata%27s_Armor_Set
https://www.bg-wiki.com/ffxi/Agwu%27s_Attire_Set
https://www.bg-wiki.com/ffxi/Bunzi%27s_Attire_Set
https://www.bg-wiki.com/ffxi/Mpaca%27s_Attire_Set
Lillith (this is one of the most common work towards really good sets people go for (especially if solo, but lillith is a hard boss so you'll still need to be good to solo her, even on ve), who are not doing the hardest content in the game)
https://www.bg-wiki.com/ffxi/Lilith
Ultimately you can tell how good gear is by just looking at it, how much accuracy or whatever relevant stats is on it. If you're building a TP set then you want to gain TP as fast as you can. So you want lots of accuracy, mutli hit, dual wield for dual wield jobs, -dmg taken and attack for white dmg. For a ws set you want accuracy, attack to overcome defence of monster, ws dmg augments (you get these from augments on 800 point domain invasion gear usually) and stat boosts that align with whatever ws you use the most
for instance evisceration
https://www.bg-wiki.com/ffxi/Evisceration
builds off of DEX so you want attack, ws dmg, accuracy and dex.
and the best weapon for it is
https://www.bg-wiki.com/ffxi/Kaja_Knife (one step below final tauret upgrade but still great)
https://www.bg-wiki.com/ffxi/Tauret
from ambuscade, which you build up from a tokko knife
https://www.bg-wiki.com/ffxi/Ambuscade_Rewards#Weapons
It's worth keeping in mind that although some gear is obviously better than others and you want to get the best gear you're capable of getting (or working towards it), the power of monsters is why you'll want the best gear and unless you're killing incredibly strong stuff you don't really need the best gear in the game.
For weapons the answer is normally either Zurim, Ambuscade or
https://www.bg-wiki.com/ffxi/Oboro
If you just hit 99 the base, starter 119 gear and weapons are usually from
https://www.bg-wiki.com/ffxi/Craggy_Bluff
https://www.bg-wiki.com/ffxi/Vesca