Horsea is one of those sprites were the cutest one was actually the Gen 1 Yellow sprite for me. There's just something about how cute and round its head is in this sprite that makes it look like a bubble floating freely in the water that gets to me, and I feel this had the best proportions. Later sprite proportions always seemed a bit off to me, and the shade of blue wasn't really as appealing to me in later gens.
That said, XY is pretty respectable as it uses a more teal blue again compared to a grayish blue in the gens from Gen 3 onwards. The animating fin brings a bit more personality and I like the way it moves as it swims.
Speaking of cute Gen 1 Yellow sprites, Squirtle has had its fair share of cute sprites. Absolutely love the shade of blue used in Yellow the most, the Crystal had such a cute smile and wagging tail and HGSS just looks so joyful! Can't resist this little critter's cuteness and that is why it was my first ever starter- even it's RB sprite was pretty respectable in terms of cuteness because it head looks so much like a bubble!
Charmander also has some cute sprites going for it, I even had to single out the 2nd animation frames for Emerald (2nd last one) and DP (last one) just because those poses are just so much cuter than the neutral pose. The eyes on the Gen 2 sprites are also precious and I really like the shading on the face in these ones. Such a shame the Gen 6 model seems a bit bland in comparison to these, and the paler lighting/colours don't really help.
Might as well complete the Kanto starter set, but I find these Bulbasaur sprites the simplest and yet the most adorable because of the simplicity. Yellow one has most personality while the Gen 5 one had the most pleasing proportions that makes the expressions on the eyes pop out more.
In spite of all those annoying Poison Stings, I really find Weedle to be quite cute. It's name is adorable and it's dot eyes and overall roundness in everything in its design makes me want to cuddle this dangerous creature. The Silver sprite really does it for me, while the almost dancing animation in Gen 6 also brings out a cute personality. Again, I'm a bit of a sucker for really nice round shapes in sprites.
Chinchou's RSE sprite just looks so happy, the pose is perfect! I just really love the expression it makes with the quirky '+' eyes and this has the best proportion in regards to the size of the lights compared to the body.
Rufflet hands down is one of the cutest birds. Look at that little fuzzball head with those cute eyes! I love the little jump it does on the animation and these small Gen 5 animations really bring out the cuteness in some Pokemon.
Leafeon's design took a while for me to warm up to but damn when I saw the backsprite I fell in love. There's just something so pleasing about the Gen 4 backsprite, maybe it's the shape of the head or the eyes but when Platinum came out, the new pose and 2nd animation sprite brought new sides of cuteness that I now appreciate. The Gen 6 model just added a touch of elegance to the pose and animation.
Vulpix is also quite adorable when the proportions are right- a simple, cute and elegant design makes Vulpix one of the most d'awww worthy Fire types. I can imagine them feeling so warm and fluffy.
Petilil is one really adorable grass type, I just enjoy the simplicity in its design. The little dress-like body and jumping animation just brings me bundles of joy for some reason.
Words couldn't describe the amount of 'squuueee' I felt when I first saw Bergmite. It's so cute with it's little triangle legs and those blank eyes hidden under a little icicle helmet/mask thing. Again, the name just adds to the cute factor, I really want to try raising one in one game- too bad it suffers from late-game Ice-type availability syndrome.
Speaking of late Ice types, Cubchoos shows how disgusting snot can be so cute. I just like how it stands there, trying to maintain balance as you see a giant dribble of cold snot swinging in front of it. Actually kind of hypnotic in a way...
Finally, just to round out the bear trio, Pancham is one cute spunky panda bear. Just a simple leaf on its mouth adds so much to the personality and those eyes are really effective in bringing out a cute, fighting spirit. Like Cubchoo, the proportion of it's head compared to its standing body makes it quite adorable.