DISCLAIMER: None of the information in this guide was made by the FANDOM staff/team; relevant credit is attributed towards the R/Battlecats Discord Server team of Trusteds and Server Contributers for research into the best Cat Units to upgrade in priority and True Form. The raw paste data is attributed towards ThanksFëanor#3087.
Server invite link: https://discord.com/invite/9anEZQQ
Below are some guidelines on what beginning players might wish to invest Catfruit into first. The highest priority units are those which make a big impact in early game because of the kind of stages players encounter at that point. These might not be the outright "best" true forms, but they are the ones that will make a difference the soonest.
By the time you need some of the long term "better" True Forms, you will have more catfruit. This is not a tier list of the true forms - many of the "low priority" units are better than the higher ones in the end, but you either can't get them until later, or don't need them early on compared to the others.
Late game use is just not considered here, as by then you can get enough fruit to TF everything, so there is no "priority" to worry over. Note that if you have certain ubers, this might not apply. If you have overpowered anti-Aliens in ITF, you have less need for Psycho or Surfer. Take the advice here with a grain of salt and feel free to deviate from this depending on your needs and situation.
With those initial points made, the lists: There are lists for Rares and Super Rares, and each come in Image and Text form, in that order. The graphical version shows more info like Fruit costs and some reasons for each placement, so it is the recommended version.
The text version is for searching and such mostly. There will not be an Uber list as they are an entirely different case, depending on what you own and such. They also cost a lot more. For uber TFs seek individual advice from other users, or refer to UDP. Collab Units are included if they cost fruit to true form. Most players won't have a lot of these so feel free to ignore them entirely, but for those who do, it is there for completeness.
Rare Units
Max Priority
Jurassic Cat -> Catasaurus
Bishop Cat -> Sanzo Cat
Psychocat -> Corrupted Psychocat
High Priority
Salon Cat -> Cyborg Cat
Rocker Cat -> Ramen Cat
Mer-Cat -> Cameraman Cat
Onmyoji Cat -> Thaumaturge Cat
Wushu Cat -> Shigong Cat
Medium Priority
Wheel Cat -> Chill Cat
Pirate Cat -> Dread Pirate Catley
Shaman Cat -> Necro-Dancer Cat
Swordsman Cat -> Elemental Duelist Cat
Rover Cat -> Catellite
Low Priority
Pogo Cat -> Jiangshi Cat
Viking Cat -> Maximum the Fighter
Archer Cat -> Cataur
Cat Gunslinger -> Rodeo Cat
Gardener Cat -> Weedwacker
Cat Welterweight Cat -> Ectoweight Cat
Collection Only
Thief Cat -> Goemon Cat
Fortune Teller Cat -> Doctor Cat
Witch Cat -> Enchantress Cat
Stilts Cat -> Acrobat Cats
Tin Cat -> Robocat
Sakura the Cat -> Dreaming Sakura Cat
Super Rares
Max Priority
Hip Hop Cat -> Can Can Cat
Weightlifter Cat -> Pizza Cat
Surfer Cat -> Seafarer Cat
High Priority
Sushi Cat -> Roe Cat
Nerd Cat -> Cyberpunk Cat
Kotatsu Cat -> Octopus Cat
Apple Cat -> iCat
Swimmer Cat -> Fishman Cat
Medium Priority
Vaulter Cat -> Housewife Cat
Fencer Cat -> The Kitty of Liberty
Freshman Cat Jobs -> Headmaster Cat Jobs
Rich Cat III -> Richest Cat
Sniper the Recruit -> Sniper the Deadeye
Orthos -> Artist Orthos CC
Hijiri Rokudo -> Baseball Mask γ
Low Priority
Bath Cat -> Luxury Bath Cat
Delinquent Cat -> Ultra Delinquent Cat
Juliet Cat -> Juliet the Maiko
Figure Skating Cats -> Slapstick Cats
Driller Cat -> Neo Driller Cat
Cutter Cat -> Neo Cutter Cat
Miter Saw Cat -> Neo Saw Cat
Baby Mola -> Citizen Mola
Aoi Hayakawa -> Baseball Mask α
Mizuki Tachibana -> Baseball Mask β
Asuka Langley -> Eyepatch
Li'l Asuka Holy Knight Alibaba -> Ghost Alibaba
Collection Only
Bodhisattva Cat -> Tathagata Cat
Cat Toaster -> Catophone
Cat Base Mini -> Cat Base Ultra
Piledriver Cat -> Neo Piledriver Cat
Backhoe Cat -> Neo Backhoe Cat