Bug: Spiders are my favorite real-life animals for the type (alongside dragonflies, making Yanmega a strong competitor), but Galvantula is too "happy" and fantastic for me so its Ariados. I also like its almost sinister Pokedex entries.
Dragon: Dragon/Poison is such a cool typing, Adaptability Sludge Wave/Draco Meteor is so cool, and its irrationally high in my favorite stat (SpD).
Electric: It's amazing how it eschews the overused fast&frail Electric model (why i also love Ampharos). Combine that with more ridiculous offensive potential with Guts and x-ray vision? Yes please!
Flying: The Drifloon line is so dopey yet so ominous (kidnapper Pokemon!) at the same time. Finding a wild shiny Drifloon in Hau'oli Cemetery as my second non-guaranteed shiny ever was a clincher.
Ghost: Even though its a mono-Ghost, Mismagius beautifly blends aspects of other types, whether its from learning Inferno and Mystical Fire or from the magical and strange nature almost reminiscent of Tapu Lele.
Ground: One single fully evolved Ground-type couldnt catch my eye enough to separate it (Flygon? Camerupt? Garchomp? Rhyperior?), but then I remembered how crazy strong / dangerous / adorable Trapinch is for its size. 100 Attack, STAB Earthquake, Arena Trap, perfect star eyes? Can't refuse that.
Ice: While I like the type plenty, overall I'm not a huge fan of the Pokemon selection. An exception is Mega Abomasnow, whose offensive potential and beastly looks alongside hail-setting are quite n-ice.
Normal: Like Ice, but less excusable since there's so many! Color Change always kept Kecleon cool, but its PMD power and great movepool boosted by Protean cut it a step above.
Poison: A long time favorite for me, from when I only had the TCG/anime and this boi in holofoil https://www.serebii.net/card/exlegendmaker/13.jpg. Man-eating plants are the best, even bland Carnivine isn't that bad.
Water: Not entirely sure why it's Pelipper lol. Maybe the design? Drizzle is always good.
Rock: Praise Cradily! Super cool design and concept, unique typing, fascinating movepool.
The others are more mainstream.