Though some subclasses in Baldur's Gate 3 can be chosen at Level 1, the Ranger chooses its subclass at Level 3. Of the three Ranger subclasses in Baldur's Gate 3, the Beast Master subclass is arguably the best choice for those who don't want to multiclass later on down the line. A major contributor to this subclass' strength is its ability to summon an animal companion.
Baldur's Gate 3: Best Beast Master Ranger Animal Companions, Ranked
Each of the five Ranger animal summons are useful in BG3, but some of them offer more utility and strength in more situations.