I’ve been making a map and their first encounter. It’s a kobald den and I’ve been stumped on the first boss. They will most likely be level 1 but maybe level 3 if needed. I want something strong that will be hard but not unfun. It’s my first time dming so I’m trying everything to make it fun. I only have motm but anything will do.

perhaps a Black dragon wyrmling with 3-5 kobold underlings with varying weapons (some archers, some melee).

I would strongly advise against multiple kobolds and a wyrmling if you intend to have them play at level 1. The damage from the wyrmling bite alone would down a character, and the breath weapon would perma-kill any level 1 character that fails the save. An encounter like that at level 3 is probably a bit more doable, but even then I'd be cautious about not having too many kobolds on the map because of pack tactics and action economy.

For a level 1 party, I'd recommend 2 or 3 standard kobolds and a Kobold Inventor for variety. Or you could just throw one Kobold Dragonshield their way. At level 1, pretty much anything is liable to be a TPK since characters have so little HP and healing capabilities. If you want to give them more of a chance (especially if you plan to have more than one combat), I recommend you hand out some potions of healing to enable them to heal each other when someone inevitably goes unconscious.

Throwing a Boss-type monster at a group of first time players @ LVL 1 might not achieve the effect of being an epic Boss-type battle.

Depending on your party size, I might throw a single CR 1 or 2 at a party of 4-5 PCs, with the understanding that the creature chosen is not a high intellect tactician, doesn't focus fire on a single PC, and doesn't act optimally. Give them a punching bag to warm up on, and maybe some well timed reinforcements should things look to be getting out of hand.

Something at the CR 3 range, like a Bugbear Chief or Hobgoblin Captain, could be used as the leader of the Kobolds - and thus be your boss. It could be reflavoured (i.e. just renamed) to be a real tough Kobold.

One that I like to use is a modified Undying Soldier. I knock down the HP to it's bare minimum (12hp), drop it's AC down to 15 by writing them without a shield. I also weaken their attack by removing their multi-attack, nerf their to hit bonus down to +3 from +5.

Doing this allows the players a better chance of hitting the soldier, and retains a really cool flavour to the enemy. As an added benefit, it's a fantastic way to teach the new players about damage resistances.

If you want it to be a real challenge and have them feel really like heroes, have some basic skeleton or zombie lurking about too and you've got a nice undead themed Lv.1 boss arena.

one tall kobold that seems to be able to double-cast spells. with a successful active perception check (or simply during melee if you get past the minions) it's revealed to be two kobolds in a long robe. the other kobolds are furious and turn on them. however, their last bid for control is to release a caged monster, wounded but furious. i'm thinking CR3 with most of it's hitpoints (but giving you the option to drop it early if it becomes deadly). something like a Basilisk, Manticore, or Owlbear. additionally, there's room for a first time ranger or druid to decide to feel out some animal handling skills and turn the tide in epic fashion.

Thank you everyone 

I’ll try to try them all in different dungeons

Just a tip: Since they are first time, maybe something that's powerful yet simple. Something like a Kobold King could work, whereas a complex Demon with lots of abilities may be too confusing for first-timers.