The Elder Scrolls XVI: Sworn to Carry Your Burdens

[Aeson]

If you use people you are reasonably expected to kill and their level as an indicator, lvl 30 (Marara and the vampire leaders) seems about it. A Morag Tong quest requires you to kill a lvl 25 character, and the main quest targets a lvl 26 character.

It's been a while since I've done it, but my recollection is that Marara's not really any more difficult to deal with than a Storm Atronach or a Hunger, and you'll be fighting those in the mid- to high-teen levels.

As to the level 26 character targeted by part of the Main Quest, if you're fighting him by the time you're of about the same level it's pretty much entirely because you've chosen to do so, because the House Hortator quests do not need to be completed to progress with the Main Quest if you're level 21+ and have 50+ renown.

Also, I would suggest that the level of the character you're expected to kill is not a particularly good guide to what level you're supposed to be when you kill them; it's certainly not a good guide to when you'll actually get a quest to kill them - the Balmora Council Club NPCs that Larius Varro wants to involve in a bloodbath are all level 9, for instance, but there's a decent chance you'll get the quest to kill them at level one or two if you follow the only quest hook you really have to pull you out of Seyda Neen at the beginning of the game and I don't think I've ever waited that long to kill them unless I just didn't go to Balmora early. There's also a bunch of hostile NPCs in Bloodmoon in the level 50+ range, several of which (level 80 Berserker Denmother, level 87 Gibbering Lunatic, level 88 Gibbering Lunatic, level 99 Insane Wanderer) exceed the nominal maximum character level for any player character,* and a number of other Bloodmoon NPCs are near the top end of the player character level range or exceed the nominal maximum level for some race/class combinations. If you include unique creatures, there's also that the final bosses of Tribunal and Bloodmoon are listed as level 100 in the Construction Set and a miniboss of sorts towards the end of the Tribunal main quest is listed as level 80.

* Level 68.5 to level 77.5, depending on how class skills, class specialization, and racial skills align. Assumes no 'tricks' (e.g. skill loss from imprisonment or Drain Skill on Self) to obtain more than (100 - [starting skill level]) levels in any class skills.

Looking for high level characters, apparently there are some non-aggressive npcs in the Dunmer strongholds (Missun atop Falasmaryon, Alfhedil in Falensarano and Abelle in Valenvaryon, all being master trainers).

The highest-level nonhostile NPC in Morrowind is Divayth Fyr, at level 65; Barenziah, in Tribunal, surpasses him, at level 70. If you include unique creatures, Vivec and Almalexia are listed as level 100 in the Construction Set.