Hunters Mark is far too powerful. You’ve basically made it a minor Sneak Attack for a class that (most subclasses for) gets Extra Attack.
It needs to cost spell slots, just like Smite, or be limited to x/day, and either way it should require concentration. That’s a lot of extra damage.
I agree with doctorbadwolf. I think Hunter's Mark doesn't need to be a spell, but it should use a spell slot to activate or be limited in a similar ways as the spell. Also, the slayer sub class isn't very elegant in the way it works with Hunter's Mark. Basically, I read it as it replaces hunter's mark and does that damage per every attack?
I would call it Slayer's Mark and say it replaces Hunter's Mark ... or just say it applies to each attack? It's very confusing either way.
Reading this Ranger ... It appears it would be the new DPR king, but that is without doing any math
The intention is that the Hunter subclass adds the Hunter's Mark damage to his attack with out having to Hunter's Mark creatures. It can still Hunter's Mark creatures in order to gain the other abilities of Hunter's Mark, just no bonus damage.
So, this is the way I approached this. First, Hunter's Mark needed to be an ability, not a spell.
Second, I'll work backwards a little bit. A paladin (The ranger's closest analog) gets Improved Smite at level 11, an extra 1d8 damage for each attack. That's already better then what this Ranger has, since the Ranger gets the same damage to each attack, but has to use it's bonus action to do so. The exception here is the Hunter subclass, which works the exact same way that the Paladin does, each adding 1d8 damage to all attacks. On top of this, the Paladin has the option to burn spell slots on Smite's (And cherry pick crits if he desires), something the Ranger can't do. It gets a little more competitive when the Hunter's Mark damage goes up to 1d10, but it still favors the Paladin.
So, at the levels before 11, the Ranger doesn't necessarily have an edge, it just works differently. Again, compared to the Paladin the Ranger does more sustained damage turn after turn, at the cost of his bonus action. The Paladin however, gets much more burst damage in the form of Smites, with no bonus action cost. It takes the ranger 4 hits (4d4) to get the same amount of damage as a single Paladin level 1 Smite (2d8). Both abilities are gained at level 2.
Also, for all classes but the Hunter, they lose any additional Favored Enemy damage, since that is now Hunter exclusive.
The Hunter loses the level 3 abilities that situationally add 1d8/reaction attack/extra attack within 5', but gains the most from the Hunter's Mark improvement.
That's my just justification for this issue. I think it all balances out nicely. I'm still open to criticism! Thanks for the feedback.