DISCLAIMER: These ratings are based on my experience, as well as advice from other players and review of forum posts. As such, information could be inaccurate, incomplete, or misleading. This is also just my opinion, and could obviously differ from yours.
Methodology: Much of my information came from the Hunter Island Forum. Other information came from this wiki, and yet more came from experience. Ratings are based on 3 factors:
Battle Power: How effective an Arkadion is in battle. This includes stats, moves, and combos with other Arkadions
Availability: How available an Arkadion is. Generally only applies as a bonus to op common Arks, since it would be a bit boring to put every Online Mission Arkadion in F Tier just because they are unobtainable.
Wildcard: Misc. tricks that can make a good Ark great, such as the Metallodious escape method or the Jabbit quick xp exploit.
S Tier
Halopard Chopperbug Metallodious Omegawyrm Guardiron Destructor Anubis Charcalynx Stegospike Frillzeon Shadowstalker Cyberwyrm Don Penguini Triphoon Kamiwyrm Ironheart Cryowyrm Angelon
A Tier
Jabbit Duskroc Searex Voltiger Raioh Ventowyrm Hydrowyrm Pyrowyrm Gaiawyrm Gremknight Nilomoth Shadowlance Luxknight Magmawyrm Typhonwyrm Arkwing Archeonis Snowja Subzeratops Vegitiger Minespider Osiriswyrm Elmoburn Jumboid Terrorfish Garudahawk Oniscar Infernowyrm Pegasion Cruiserfish Goldius Lavagant Octoneer Roidguard Golgemios Scorpionix Gearwolf Naga Goldenhorn Coldheart Hadehound Chillqueen Haniwel Magmahorn Minoblast Mossgolem Mandaroot Gigarock Bloodclaw Astroleon Meowzard Stratowyrm Felimancer Hydrablazer Vortexor Plasmorex
B Tier
Stegotops Blazerunner Bouldon Dullakhan Mammut Gigaceros Fireheart Nightmare Cherubion Sanctolamb Bearfist Marspin Mechadino Inferneo Skywolf Greenking Twigster Hammertooth Volcawolf Toucan Flamogun Georex Raptorex Leviathan Pyroviper Necrodrake Scorpionix Thunderback Brakitorus Soltusk Glazio Pengboss Skullrex Stratowyrm Spinoflame Rooknight Hydrablazer Shisaguard Ninjagale Naga Barricadus
C Tier
Goldram Bosshorn Stormfox Orcoid Solama Tombguard Skullwraith Volvoxon Firequeen Flamorider Nitrobird Raijin Tremorback Dreadwolf Sledgeskull Vulcaraptor Titanwyrm Moji Snowstag Tidalwhale Nightmare Gryphon Saphireon Kentucky Rockoid Prometheo Nightmare Krakhan Narlance
D Tier
Breezehawk Spooker Freezebear Scorchtail Rockoid Blazewing Joustfish Moogong Prometheo Orcow Puffoxin Plasmodious Seaspine Firesoldier Shrubtooth Glaukindle Grizzrex Solama Knighthawk Sabreshark
E Tier
Bluetail Biteschool Mantisword Beowolf Frostknight Pterosoar Prickster Volvon Hedgeheat
F Tier
Remus Pharaohcat Hedgeheat Yetigo Megalorex
Outliers
Despite Seaspine being c tier, its earlier evolution, Seaquill, is actually in A tier for being arguably the best early game arkadion in the entire game
Bronzeshell has an ability which allows it to transform into almost any other Arkadion, including some arkadions that were never even officially released. This ability is so random that it cannot be accurately ranked
Frostjack is great for fusions despite its evolution being terrible for everything
Gargol has the same problem as Bronzeshell
Cherub has a similar situation to Frostjack, though its evolution is still pretty good
Analysis
Just looking at the tier list, you’ll notice that the majority of monsters are clustered around A, B, and C tier, with A tier being the most common. The main reason for this is that Hunter island is quite well balanced, and lower starred monsters aren’t that much worse than higher starred ones. Those of you doing your research may be surprised that one of the S tiers is only 4 stars. This is because, in a game like Hunter Island, raw power doesn’t really matter that much. However, the reason that A tier specifically is so packed is actually for a different reason. Looking at A tier, it is noticeably different, and not just because of its size. It is filled with egg monsters and Online Mission monsters. The reason that so many of them end up in A tier is because the devs needed to motivate people to participate in the online missions, and to buy gold for golden eggs. So, they made sure that egg exclusives and Online Mission monsters were strong, but not strong enough to be too dominant, or to be must-haves, because only about 6% of players(who participated in the online mission) actually got each Online Mission monster, and egg exclusives were even rarer. If you exclude egg exclusives and Online mission monsters, then the tier list looks more or less like a standard bell curve.