The starters are all very easy to get, as they're gifted to you. All are extremely good for playthroughs, and Popplio is probably the best.
Abra can be found early on, and is fairly easy to catch (Teleport can be annoying though). Kadabra and Alakazam are both excellent Pokemon for an in-game playthrough.
Magnemite is a good Pokemon both defensively and offensively. Magnezone unfortunately cannot be obtained until much later, but Magneton carries its weight nonetheless.
Growlithe has been a consistently good Pokemon for all generations. Arcanine does extremely well in Alola, and Growlithe is relatively easy to find and catch.
Magikarp, while awful at first, can be truly fearsome as a Gyarados. It can be caught easily, and while it may take a bit to level up, the results are well worth it.
Bounsweet evolves into a very reliable physical Grass type, Tsareena. It's easy to find in the Lush Jungle, and doesn't evolve overly late either.
Sandile eventually becomes Krookodile, one of the best Ground types (and one of the more easily accessible). It has a large movepool, and amazing Attack to go along with it. It's obtained somewhat late, though.
Hope I helped!