Highlander decks are unique in a way that you can go to the Mulligan Guide stats on HSReplay and just see what cards are good and no need to thinking do you need 1 copy or 2, because you cannot have 2 copies anyway, so card is either good and in or not... A week or so ago I looked at current stats on Secret Highlander Hunter Archetype as a whole at Legend-10 and came up with this 30 cards list which seemed to be best performing. It turned out to be a mesh up of what seems at first as mostly broken synergies, but it worked out pretty well I went on win streak from Rank 5 to Rank 3 and then From Rank 3 to Legend I wasn't winning every game but kept pretty high winrate still and I am suprised that even though some cards don't seem like a good fit with each other were still good enough to actually matter a lot.

Main questions you might be wondering about:

1. Why run both Phase Stalker and Subject 9
   Yes it was indeed experimental deck on my side. But I am highlander having this redundancy actually made sure to thin out my deck every time. And even if I draw both I just drop Stalker first and then when he inevitably gets removed just drop Subject to draw the remaining secrets. And in a few cases when one of them draws nothing, well it doesn't usually matter all that much because you have stuff to play rather than top decking secrets.
2. Is only 5 secrets enough?
   Definitely. Other secrets are just bad and I wanted to run only good cards. It removes some value from Subject 9, but on the other hand it also reduces chances of Subject 9 milling you.
3. Why Corrosive Breath with only 3 dragons?
   I mean I just included because it was in top. What I found is that well if you don't get upside of 3 damage to face it is no big deal you mostly need it for removal usually. 3 damage to face isn't what usually matters, and if it is what matters you can wait until you draw dragon or just 2 hero powers later you win anyway.
4. Why Tracking? I don't like burning cards BabyRage
   Burning cards doesn't matter, if you never reach fatigue. Just imagine those cards are bottom of your deck, same difference. You never draw your whole deck, so missing 2 cards which you for sure know you will never draw actually is better than just hoping to draw something and never get it.
5. Is Spellbreaker worth it?
   Totally, there are plenty of cards to silence even outside of Rogue, and can be always used to silence a taunt for lethal.

Mulligan Guide

Try to get a curve pretty usual stuff... Keep 1 drop (Don't keep Tracking though, the actual minions only), a 2 drop (best one being Phase Stalker), 2 drop can be a secret, though only good secrets to keep early are usually Rat Trap and Snake Trap and maybe Explosive Trap if you expect aggro opponent, [Hearthstone Card (zephrys) Not Found] is also a nice keep. Keeping a good 3 drop should be also consideration (usually Animal Companion or Ursatron are good)

On a coin consider coining out Phase Stalker. Or Keeping Hyena Alpha if you have a secret that won't get activated (usually Rat Trap or Snake Trap) to coin Hyena on 3, which is pretty strong play.

Also Tech keeps would be Desert Spear against expected Aggro or Dinotamer Brann if you expect slow opponent.

Keep Spellbreaker against rogues for sure.

Card replacements

You must have all 3 highlander cards those are not replaceable. But other than that:

Mecharoo to replace a 1 drop

Snipe to replace a 2 drop

Eaglehorn Bow and Deadly Shot and Dragonmaw Poacher to replace mid drops

Siamat and Unleash the Beast to replace big drops

if you included Siamat mid drops can also be replaced with Sandbinder

Example games

Handlock: https://hsreplay.net/replay/Aaiif69FjmCXsKCTGuadcc you are the SMOrc deck here

Combo Priest: https://hsreplay.net/replay/6dXCne9VfBdwLLkejKogwF don't let them stick a minion

Highlander Rogue: https://hsreplay.net/replay/t8xTh6e3Gmge4CuWcZbcV6 Deny them tempo while SMOrcing as much as you can so that they die before they can teck advantage of their value.

Treant Druid: https://hsreplay.net/replay/DcJaztYHKULUL29FkYvgXg Don't let them stick minions especially treants.

Highlander Mage: https://hsreplay.net/replay/NhcEAAuTgfvEyBSqEALp3o Just SMOrc them down before they do big plays

Pirate Warrior: https://hsreplay.net/replay/JgvyN7wBTWmPRKEG4T24dP Same as always don't let them have minions.

Galakrond Warrior: https://hsreplay.net/replay/nPzJhTSpKwu2nCQsZmZyPe Match up is hard, but basically you need to somehow get more value than them. Yes this super lucky win when I drew all of my pay offs is pretty much it... Or you have to win tempo by so much they can't outrun you: https://hsreplay.net/replay/U6x6mmfe3rYfNHyCcucnRd

Mirror: https://hsreplay.net/replay/ZSs2dW6NmdidBA7aYS6QPd guess what you cards on average are slightly better, so just try to win tempo battle

Face Hunter: https://hsreplay.net/replay/Yai87AmVPMFb98LBx4rdMa suprisingly enough you go face better than they do quite often. Be aware of Unleash the Hounds + Timber Wolf combo though (That is the only way I died against face hunters)

Galakrond Shaman: https://hsreplay.net/replay/35CVhwkDFtW9UegFjoYQtg Just try to SMOrc them fast enough so that they are dead

Deathrattle Rogue: https://hsreplay.net/replay/jsowpQx3pziqCswwtgPtsR Kill them before they get to play the big stuff, and also Spellbreaker is really good here but don't just use him unless he is relevant.