I think good shinies are always subjective. We all like different thing. But a few of my favourites that are catchable in USUM (i.e. not egg hatches) are:
The Pikipek line - I love the pink on Pikipek, and the multi-coloured beak improves with each evolution
Zorua/Zoroark - Blue is my favourite colour, and it looks so good as the accent colour on Zorua. This changes to purple on Zoroark, which also looks great.
Braviary - The changes are a bit more subtle, but I think the colour scheme looks better than the original
Oricorio (Red) - The inversion of the red and black looks great. And you have an added bonus of being able to change the formes using nectars.
Ribombee and Liligant - I am a sucker for pink shinies, and the addition of pink works really well on these ones.
Dewpider, Lurantis, Stufful - I just really like these ones
Phantump - I think the grey scheme makes it look more like a ghost, and really suits it.
There are also some awesome shiny legendaries you can hunt in the wormholes - Ho-Oh, Lugia, Latios, Latias, Kyogre, Mewtwo, Xernea, Rayquaza... are all amazing, and worth the effort of soft resetting.