There's not actually that many forced encounters, but here's the list: (only includes the ones from the main storyline, so Episode RR forced encounters won't be included)
Starter (Rowlet, Litten, Popplio)
Pinsir (must encounter for Alolan Exeggutor to appear at the area with the Sun Flute/Moon Flute and allow the player to climb its neck to get the flute)
Type: Null (not really an encounter, more of a forced gift from Gladion after becoming Champion)
Necrozma/Ultra Necrozma
Poipole (it's guaranteed that you will be offered Poipole, but you don't have to take it)
Source