Here's a rough estimate of where each type stands in Monotype LC:
S tier:
Water - Water is an incredibly versatile type, being able to run bulky offense, offense, Sticky Web, Double Shell Smash and many MANY different kinds of Pokemon. Chinchou can completely check Electric-types for the team, and Skrelp, Mareanie, and even Wingull can take care of Grass-types. Water's main stop is Dark teams, as Surskit isn't really a viable check, and things like Scraggy and Vullaby can eat a Scald and sweep teams easily. Against other good types, namely Fighting- and Normal-types, Water teams have a very slightly favored match-up, as the former is stopped by Mareanie, Skrelp, and Slowpoke, and the latter is stopped by the myriad of Rock-types (Kabuto, Tirtouga, Omanyte) and certain other Pokemon such as Chinchou and Shellder having pretty good matchups against Normal-types. All in all, Water is the best type in LC Monotype. No question.
Dark - Dark's ability to easily break through its checks makes it another great type in LC monotype. There's not much else to it, Dark- just has a good matchup against anything, bar Fighting-, Flying-, and Normal-types, which it is in a very slightly bad matchup against. This is because Dark's Normal-type check is Scraggy, which is easily worn down by Flying-types such as Fletchling, Taillow, Doduo, Rufflet, Pikipek, etc. Flying-types are checked by Pawnaird, and with Flying-types using a lot of Fighting-type moves nowadays that isn't the most solid of checks. Fighting-type checks on Dark teams consist of Vullaby, Grimer-A, and Stunky, the former being a big issue, but the latter 2 being easily worn down by Fighting-types. Conclusion: Dark-type is pretty hard to beat.
Flying - Similar to Dark-types, Flying-types can easily wear down its checks. Things like Rufflet's Superpower, Doduo's Jump Kick, and Archen's Earthquake all wear down checks. The only real good type against Flying-types is Water as it has many Rock-types and Chinchou. That's basically it.
A Tier:
Fighting - Fighting has always been towards the forefront of MonoLC due to Mono-Fighting being a popular team on the ladder in mid-late XY. Not much has changed since XY, but Fighting now has many worse matchups. Flying, Grass, and Poison are all pretty bad matchups for Fighting. However, Fighting-types certainly have the ability to wear down checks with Knock Off, Ice Punch, Acrobatics, etc.
Normal - Normal-types don't really have many counters since Munchlax can Pursuit trap Ghosts and many Normal-types carry Knock Off. The many Flying-types Normal has also help with taking down Fighting-types. Normal's main problem however is that it doesn't do well against very bulky teams (Water, Poison, and Grass being issues if using bulky teams.
Grass - Grass's combination of bulk and offense makes it pretty good in general. However, sun teams aren't that great and it lacks the ability to beat most of its checks such as Rock, Fire, and Steel. That's about it.
Rock - Rock has access to many great Pokemon and can check its checks. Haven't really used this type so can't say much more, although its still decent.
Poison - Poison-types easily take down their checks: Psychic-types are Pursuit trapped by Stunky and Grimer-A, Ground-types are checked by Foongus and can be switched in on with Gastly. However, Diglett is an EXTREME issue, and so Poison still is pretty bad against Ground.
B Tier:
Ground - Ground makes pretty good sand teams, but its lack of variety and large variety of checks keeps it from being very good
Fire - Just like above, but Fire doesn't have any good abusers besides Ponyta. However, Fire has access to Torchic which can make a very good secondary wincon. Doesn't make it very goo though
Ice - Same deal, except it is atrocious outside of hail. Ice is nearly C tier, but hail's viability as an archetype keeps it barely good enough.
Steel - Steel lacks many good Pokemon, and as such it is extremely predictable. Its Pokemon also lack any sort of synergy, making it predictable and weird to play with somehow. However, it does have some very good Pokemon, so it can be used.
Ghost - Similarly to Steel-types, there aren't many good Ghost-types in LC. Additionally, the myriad of Dark-types in LC mean almost every good type has its own Pursuit trapper or fast Dark-type to completely beat Ghosts. However, Ghost-types have decent offensive synergy, so Ghost is still pretty usable.
Bug - Bug has access to some pretty good Pokemon, but not enough. It can use Sticky Web, U-turn, and other offensive archetypes decently, but other types, such as Water out perform it in this way.
C tier (a.k.a. unusable types):
The follow types have a lack of good Pokemon and can't beat any of their checks/can't beat most types in general
Fairy
Psychic
Electric
Dragon
I'm not really a monotype player, but I do play a lot of LC, and those are my impressions.