Units can be upgraded in 5 ways. Experience, Pieces, Skills, Orbs, and Crowns.
Upgrading your characters is expensive. Thankfully, not everyone is worth spending experience on, and that includes characters you are using. I would divide characters up into 4 groups.
Characters who hit things
These are characters who will be punching for you. Generally, your best damage dealer in a given element. You can cover pretty much every enemy in the game outside challenging event quests by just upgrading 3 damage dealers of either Fire/Dark/Water, or Air/Earth/Light. This trio would complement each other, with someone who is super effective against everyone you will encounter. Upgrade this trio to level 80, and any other you may want to use to level 60-80 as needed.
Examples include: Saya, Mai, Tsubaki, Any SS attacker not named Meifang
Characters who get hit
These are your tanks. Whether its a character classified as a tank in game, or just someone with a pile of health or defensive tools. They are going to want a lot of health and defense so that they can survive a beating. You will not need this until late game, since the character who hits things will double as the one getting hit. You may use 1 of these on a team, maybe 2 if they are also a powerful support. Upgrade these characters to level 60-80 as needed.
Examples include: Hibiki, Drei, Kokonoe, Fuzzy
Characters who don't hit or get hit
These are characters who exist on your team purely so that they can use their skills, passives, and maybe an S or DD for its effect. These characters you only upgrade enough so that they do not explode when hit, and are mostly made up of supports and healers. They will typically fill out the rest of the slots on your team. Upgrade these characters to level 40-60 max as needed.
Examples include: Ciel, Kajun, Kuon, Kokonoe
Ranking Damage Dealer
This is end game stuff. If you want to participate in end game stuff, this is the only character in the game you will ever max out at level 100. It is the damage dealer you are using in Ranking events, where you are given points based on how much damage you do. Maxing out their stats means you max out their potential. Upgrade these characters to level 80 minimum, 90 recommended, and if you plan on competing for the highest ranks, this is the only character you will max out at level 100.
Examples include: Jubei, Kagura, Another Dark Mai, Es
AI Pieces are mostly farmed by buying them from shop. This is a very good use of you "buy from the shop" daily until you have most of what you need.
The Target is what you should be farming to before moving on to someone else. Any character listed as "Stats" for their tier is the lowest priority in their tier. You only farm shards to upgrade their stats, since they don't need any of the other perks that come from upgrading their rarity.
NOTE: If we have a character raising campaign for an AI, just do it once for the Crystallized Azure and then never again. It costs too much AL to farm it daily, and that is better spent on SS pieces.
Don't farm S Character Quests. Do them once for the Crystallized Azure, and then never touch them again. You will accidentally max these characters by just rolling in the gacha, and you might max out half of them before even getting your first maxed SS or AI.
The exception is Academy Noel. Academy Noel impossible to max out unless you farm her character quest. But she is also terrible, so just do it if you feel like you have free time to do so.
These are the characters that you would actually want to farm. This is a decent use of your extra AL in any given day since you can only do 3 a month. Personally, I prioritize limited characters first, then do others.
NOTE: If we have a character raising campaign for an SS, you should probably farm that. You will get, on average, 75 pieces per week during one of these campaigns.
Some characters may be listed multiple times. That is because they have some essential skills they need early, and some less essential skills they get later that are still worth focusing on.
Also, the characters listed under the target SS+ are ONLY for the Ranking game mode. At SS+, you unlock the ability to upgrade your astral, which is currently the best form of damage in these modes.
I did not include stats rows here because that would just be cluttering the screen. Refer to my Tier List for that. Generally, its just characters who hit things or get hit, as well as obviously your Ranking Damage Dealer.
Crowns give a minor stat boost. You normally get these from dupes, but you can also rush them with exceed emblems. The REAL reason you want to farm crowns is because some characters have costumes they get at certain crown levels.
I would not recommend using exceed emblems for anything other than costumes. If you want the minor stat boost... It is only worth considering it on Limited characters who are also your Ranking Damage Dealer. Also, if you want to spend them, spend them on Bronze first and hope for rerolling after. They get more and more expensive at Silver and Gold.
I'll make a list of these characters that looks nicer later.