I am a DM and I have created a group of Rangers that serve as guardians on the frontier against the Orc Tribes that are nomadic.  The Orcs resent the humans pushing into their historically tribal lands so the Rangers keep an eye on them.  Most of the NPCs that belong to this group are Human Gloomstalker Rangers.  The Rangers, called Men of the Wilds, travel in groups of four to six and mostly spy on the Orcs to detect anything that looks like a buildup before a raid.  They also kill Orcs that venture into the buffer zone the Rangers use to prevent them from scouting out the area and finding an efficient route to use for a future raid on the subdued human lands.

A party leader will typically be a level 6 GS Ranger, with a level 5 GS ranger and two level 4 GS rangers to fill out the group.  A training team may also have two level 3 GS rangers with them.  These groups are only attempting to help the Level 3 fellows reach Level 4 safely.  These rangers are split about 50/50 with archer builds and melee builds.

So, what are the best spells for the level 4, Level 5 and Level 6 builds?  Also, what would a Level 10 build look like, because that is something the rangers wish to attain, so making a build that doesn't get you in the right shape to attain the level 10 build wouldn't be part of their progression.  At Level 10 they would be serving as a large Group Commander.

Right now, the Rangers are using this build in my NPC template ...

Cantrip:  Minor Illusion

Level 1: Disguise Self, Hunter's Mark

Level 2:  Rope Trick, Spike Growth, Pass Without Trace

In my homebrew world the kingdom supports the MotW by equipping them and rewarding them.  Each Ranger carries three +1 blades; a Falchion, a Short Sword and a Dagger.  They also carry a +1 Longbow.  They are given at the start of each deployment, a bandolier with 10 weak Potions of Healing, 2d4.  Drinking one of these potions requires a bonus action.

There are a few MotW that are not Rangers and a few that are not Humans.  So you would find Fighters, or Clerics, or Druids among their ranks, but these together amount to 20% of all the Exploring Rangers.  There are others that serve in the various supply camps that support the rangers.  Among these are cooks, animal handlers, blacksmiths, cartwrights, etc.

So, if the PHB, XGTE are available for spells, what would you recommend for the builds?  Would you recommend something different for a melee build as opposed to an archer build?

Thanks for your interest.

Though it’s tougher to manage as a DM, I’d suggest they would likely have different, complimentary spell lists to help cover for each other. For example, if one person on the team has Pass Without Trace, no one else needs to have it. That would free up slot space for someone else to have something like find traps, while another has silence and another has lesser restoration.
Also, for sneaky, fast moving spies, why are they carrying so many swords each? I could see the melee types carrying two if the intention is that they dual wield, but it seems like the third would just be extra weight. And for the archers, probably just a short sword or one other finesse melee would be enough, since presumably they’ll have archery fighting style and not dual wield. 

i found these three spells very handy.

fog cloud: good if you need a distraction or cover.

healing spirit: this works great with archers, you use your attack then use a bounes action to heal others while in the fight. (it saved a few party members from death rolls as well.)

zephyr strike: lets you get extra movement, avoids opertunity attacks and deals a d8 of damage.

Maybe have one with Goodberry. Cast it before the group rests. can be used instead of rations or if the group doesnt have time to forage plus it is a hit point if at deaths door. Not everyone would need it though.

People under rate cordon of arrows - Especially in an all ranger team because they are more willing to setup good ambushes. Rangers that also can get ready access to creature poison can then use the poison arrows on their spells and traps (caltrops, bear traps, cordon of arrows, swift quiver etc).

beast sense(on a blind sight creature) is a unique option for a party member where someone else casts fog cloud or darkness.  This allows the full use of attacking an enemy with advantage while they have disadvantage.  

Having ritual caster druid on at least one ranger really opens up some spellcasting tricks (like possibly getting beast sense earlier)

In my experience, Absorb elements is a very powerful defensive option, as you get resistance on demand without items or preparation. Since elemental attacks are more common at higher levels the first level slot becomes more useful the further you get.

As Xalthu said it’s probably best that the spells be spread out. This would be better if the UA2 ranger is used as they have prepared spells not known making it far easier to explain having different spells.  Given that, just using the PHB I would suggest the following:

L6 Ranger: 4 spells known (PHB): L1: hunters mark, goodberry, alarm; L2: pass without trace; prepared (UA2: 2C, 4L1,2L2): message, guidance, hunters mark, goodberry, alarm, detect magic, pass without trace, silence
L5 Ranger: 4 spells  known (PHB): L1: hunters mark, speak with animals, fog cloud, L2:spiked growth; UA2 - prepared 2C, 4L1, 2L2:mending, spare the dying, hunters mark, speak with animals, fog cloud,  healing word, spiked growth, gentle repose
L4 Ranger (1): 3 spells known (PHB): hunters mark, hail of thorns, fog cloud; UA2 (prepared: 2C 3L1): poison spray, shillelagh, hunters mark, hail of thorns, zephyr strike
L4 Ranger (2): 3 spells known (PHB): hunters mark, longstrider, cure wounds; UA2 (prepared, 2C, 3L1): poison spray, shillelagh, hunters mark, longstrider, cure wounds