I have read a lot of threads in here, the gloomstalker/assassin/battle master seems to be the most revered combo. I want to do something different. My ranger is 2nd level now and I am struggling on the multiclass combo I have in mind as to when to take what level. I was heart set on swarm keeper for flavor from the beginning, i recognize the benefit of battle master but i think i prefer echo knight at least to level 3. Then rogue simply for the sneak attack dice on a longbow. So may end up 5/3/12 if it plays out to 20. 
  The backstory. Custom lineage with magic initiate, background: acolyte. He was raised in a church of anubis and kinda wants to be a righteous slayer of the undead. (Goodberry, druidcraft, mending on initiate) Feels like for the story mode he needs some cleric levels but not sure if there is a subclass to compliment archer while smiting undead. And extra attack… my math puts it at 9th level or later, means i need to rush steady aim or delay multiclass till level 5.

Not married to any subclass, just like the mobility on echo night. Was going to roll one of those instead of ranger but this guy fit the party better. We got a barbarian may go bear or wild magic and an undead lock. I may be using all my slots on cure wounds instead of hunters mark. Probably should dip hard on cleric…. 
   Str 14 dex 18 con 15 int 12 wis 16 cha 10

so no bard…

Do you know a very good multiclass often underrated to Rangers? Cleric. War Cleric.

You get access to great spells like Bless and Healing Word. Bless is really great if you decide to go on Sharpshooter route. 16 WIS gives you 3 extra bonus action attacks per long rest (War Priest), which is kind of meh, but could be useful in the right situation.

Personally speaking, I don’t like Assassin sub-class and I don’t think this triple-class is the most effective build out there. Gloomstalker 8 / War Cleric 2 can give you a basic setup of four attacks in the first round (Dread Ambusher + War Priest), probably with advantage (Umbral Sight) but if not, Bless + Archery FS can mitigate Sharpshooter penalty to hit and you can use your Channel Divinity (Guided Strike) as a last resort. Simple & effective.

Nice

I like going heavy in ranger and swarm keeper is pretty good as is. 

some things to think about. classes that can dash (BA) will work on the limited fly fly speed. 

The beast master (Both phb and Tashas) or the new drakewarden Kind of allow you to be in two places at the same time similar to eco knight.

Monks could add some interesting mobility to the class if you only want the occasional boost. A mostly ranger with a dip in monk has some tactical options as long as you pay attention to resources. 

I don't like 3-class Multiclass concepts... it's really hard to get the most out of it. I don't think you'll get a lot out of Echo Knight in this build concept... Your Swarm already gives you some solid mobility, especially when paired with a Rogue's Cunning Action.

If it was me, I would go Ranger Swarmkeeper up to level 5 just to get Extra Attack, then put the rest of my points into Scout Rogue. It also gives you even more mobility, since you can use your reaction to basically get a free disengage from enemies who get too close to you, and you can double down on your Ranger skills by gaining expertise in Nature and Survival. Beyond that you're mostly sticking with straight Rogue going forward mostly for the boost to your Sneak Attack, but the other Scout-specific skills are still pretty useful to your build. 

It depends on what you want to focus on... if you want to prioritize damage in combat, then going mostly Rogue would be best. But if you just want Sneak Attack for a quick bonus to damage and want to get more versatility with spells and exploration, then I'd treat Ranger as your primary.

Thanks