Raticate: Both are garbage Pokemon. But the Alolan form of Raticate is better because it gets Dark type STAB, while regular Raticate is just outclassed by many better aggressive Normal types.
Raichu: Regular Raichu is mediocre and largely outclassed by other Electric types. Alolan Raichu has access to Surge Surfer, which makes it a powerful force combined with Tapu Koko. Its Psychic type also allows it to benefit from Tapu Lele.
Sandslash: Regular Sandslash is outclassed as a Sand Rush sweeper and Rapid Spinner by Excadrill. Alolan Sandslash has a unique typing and is one of the few sweepers available to Hail teams, making it the better Pokemon.
Ninetales: Alolan Ninetales has higher Speed than regular Ninetales, allowing it to outspeed base 100 Pokemon, which is always an important speed tier to hit. Fairy typing is a very powerful type, giving Alolan Ninetales better resists and immunities than regular Ninetales.
Dugtrio: Alolan Dugtrio is worse than regular Dugtrio. Regular Dugtrio has access to the Arena Trap ability, one of the most powerful abilities in the game. It's also faster than Alolan Dugtrio, which is a big deal considering its stat distribution. The slightly higher defense of Alolan Dugtrio does less to help it survive than being able to take the foe down first.
Persian: Regular Persian is outclassed as a sweeper by a ton of Pokemon. Alolan Persian isn't great either, but access to Fur Coat makes it decently bulky. The big draw for Alolan Persian is access to the rare move Parting Shot, which when used as a Z-Move ends up fully restoring the HP of the Pokemon that switches in, making Alolan Persian a solid pivot.
Golem: Both Golems are bad Pokemon. However, regular Golem is just so much worse than other Rock/Ground types (such as Rhyperior and Rhydon). Alolan Golem is a unique typing, which always makes a Pokemon more useful due to unique resistances. Alolan Golem has access to two great abilities in Magnet Pull and Galvanize. Galvanize in particular gives it very powerful hits off of Explosion.
Muk: Alolan Muk is much stronger than regular Muk almost exclusively due to the fact that Poison/Dark is such an amazing defensive typing.
Exeggutor: Another case of both forms being bad Pokemon. Alolan Exeggutor is better because it has a slightly better movepool. It hits hard with Draco Meteor and has additional coverage with Fire type attacks. It can also run physical attacks to provide coverage better than regular Exeggutor can.
Marowak: Alolan Marowak has an incredible typing that puts it leaps and bounds ahead of regular Marowak. Fire/Ghost provides some great resistances and immunities, especially when aided by Lightning Rod, which is useless on regular Marowak but provides an additional immunity on Alolan Marowak.
So all of the Alolan forms are better than their regular forms with the exception of Alolan Dugtrio, who is way worse than regular Dugtrio. Most of them aren't notably stronger than their regular forms, however some like Marowak and Muk greatly benefit. Mostly their increase in power is due to a better type, or in some cases due to a better ability. To a small degree the change in stats helps (although really only for Alolan Ninetales) and access to a superior movepool (as in the case of Alolan Persian.)