What is your character's background? For your first level Ranger abilities, I'd suggest picking a favored enemy and terrain that jives with your background rather than strictly focusing on potential optimization for the particular campaign you are playing. Especially given the text for the ability: "Beginning at 1st level, you have significant experience studying, tracking, hunting, and even talking to a certain type of enemy." Your background is one reflection of that experience. You might be pleasantly surprised how useful they'll be anyway even if they appear more random at first. Or perhaps they won't be mechanically useful at all and your character will need to learn and grow to overcome their "stranger in a strange land" dilemma - good story fodder.
That said, as you explore the setting, your Ranger is learning about the common enemies and the terrain. The 6th level additional favored enemy text reflects this: "As you gain levels, your choices should reflect the types of monsters you have encountered on your adventures."
As an example, I recently created a Shifter Ranger with the Fisher background. Her favored enemy is beasts and favored terrain is Coastal as I thought those were thematically appropriate for the character. Certainly not optimized for our upcoming Curse of Strahd West Marches campaign, but true to her background. Then again, I can see how beasts will come in handy and, if the DM is lenient, maybe Coastal will be useful around Lake Zarovich. I'm gonna go out on a (Strahd zombie?) limb and say undead as a favored enemy is likely going to be quite appropriate at 6th level.
Background is archeologist. I have a start on it - The character is from the north, luskan area, I have not completely made up the backstory yet so I can tailor it to these things, that is what I am trying to do. For favored terrain, this is the only adventure I will use this character in so I think anything other than arctic is a waste. For the character I can't use any UA. It is a PHB ranger with the subclasses from XGE.
In the past playing with this DM, a Ranger in the party is a necessity in wilderness adventures. Playing Tomb of Annihilation we spent the first 2 levels being lost 80% of the time despite high survival skills. I had a rogue who I did not intend on multiclassing who had a 13 wisdom (rolled). When we made 3rd level I multiclassed into Ranger solely to avoid getting lost any more (Rangers are never lost in favored terrain).
In the games we play the "never lost" feature is for sure the best benefit of the class. With that in mind I think arctic is the favored terrain I want. I just need to figure out the favored enemy and then flesh out the back story.
As an archeologist I think I was a treasure hunter employed by the Drow band running Luskan. What kind of creatures did I run into robbing tombs/ruins/kingdoms for the Drow? Could be the
"undead" who were buried in the tombs I was exploring? Could it be
"monstrosities" like Yetis, winter wolves and Rehmoraz that seem common to the far north? Maybe I was competing with other
"human" explorers or robbing ancient abandoned dwarven kingdoms where I learned a lot about
"Dwarves". Or maybe my favored enemy are the
"Gnoll" and
"Hobgoblin" mercenaries in Luskan because they would harass me on the way to and from the city, or maybe I was not employed but the Drow but rather enslaved by them and
"elves" are my favored enemy. Those seem to be some obvious potential thematic choices, but I want whatever I choose to be somewhat useful in the adventure.