Sinnoh is definitely my favorite region when it comes to all the new Pokemon introduced there. There are many cool Pokemon from there: namely Gallade, Garchomp, Roserade, Shaymin, Darkrai, Dialga, Palkia, Giratina, Arceus, Porygon-Z, Buizel, the starters, the Lake Guardians, the Luxray line, Staraptor, Dusknoir, Abomasnow, Magenzone, Glaceon, Electivire, Lucario, Weavile, Cresselia, Rampardos, Drifloon, Cherrim, the list goes on and on. Many of my favorite Pokemon are from Sinnoh, and there are very few Pokemon I don't like from there.
Hoenn is a close second. Yet again, most of its new Pokemon look pretty awesome: memorable Pokemon include the starters, Salamence, Metagross, Absol, Milotic, Aggron, Skitty, all the legendaries, Roselia, Beautifly, Nuzleaf, Mawhile, Sableye, Ludicolo, Shiftry, Altaria, Zangoose, Manectric, Wailord, etc. Much like Sinnoh, almost all the new Pokemon stick out to me in a good way.
Both Alola and Kalos didn't have many new Pokemon compared to other regions, but I like how they want for "quality over quantity" when it came to both regions. All the new Pokemon in both regions don't stand out to me compared to Sinnoh and Hoenn, but I still enjoy most of their designs for the most part. In Alola, some of the Pokemon I really like design-wise include Lurantis, Lycannroc (Midday), Primarina, Decidueye, Toucannon's evolution line, Silvally, the legendaries, Ribombee, and many more. In Kalos, I really like Greninja, Tyrantrum, Sylveon, Aegislash, Braixen, the legendaries, Talonflame, Gogoat, Trevenant, Noivern, Barbaracle, etc.
Aside from Chandelure, Bisharp, Samurott, Serperior, the basic stages of the starters, Meloetta, Zebstrika, Gigalith, Excadrill, Scolipede, Krookodile, Hydreigon, Volcarona, Golurk, Haxorus, Reshiram, and a few others that I don't remember off the top of my head, Unova's Pokemon designs don't stand out to me compared to the other regions (I guess that's mainly because most of the region's Pokemon felt more like knockoffs of the Kanto Pokemon), but I still liked their overall Pokemon much more than Johto and Kanto's selection.
Meanwhile, the Johto and Kanto Pokemon don't stand out that much compared to other regional Pokedexes, at least in my opinion. In Johto, the only Pokemon that I really liked based on design were Donphan, Umbreon, Espeon, Skarmory, Scizor, Lugia, Ho-oh, Suicune, Noctowl, Chinchou, the Ursaring line, and the Ampharos line. In Kanto, its even less. The only Kanto Pokemon I really liked based on design were Gengar, Vaporeon, Jolteon, Ninetales, Onix, Hitmonchan, Hitmonlee, Alakazam, Machamp, and that's pretty much it. Between Kanto and Johto, I like Johto's selection of Pokemon more.