Warrior starting class is pretty good too but yeah Knight might be overall best for new players. It has the 100% physical absorption shield, beefy armor and great stats plus very solid all around starting weapon. Herald is not bad at all either starting out. You get an extra heal spell, you get a long ranged spear weapon, nice stats and a very solid shield too.
I personally think that for Dark Souls 3, it's best to start out learning how to roll well and learn to play faster weapons with dual wield or 2 hand. So I personally say Assassin would also be a good starting point for people who want to focus on Dexterity style builds and not rely on shields too much. You will struggle early on but you will learn the game faster and not rely on shield blocking so much (which unlike previous Souls game, in 3 you get punished a lot for relying too much on it).
Ultimately it's more about what kind of playstyle and build you want.
