I like playing a Blaine deck (i.e., Rapidash, Ninetales), so I started my ranked season to see how far I could get with it.
I started at the bottom of Ultra Ball 2 on 510 points. Forty-six battles and probably about 4 hours later, I finished at the bottom of Ultra Ball 2 on 521 points, and that's where I decided to end it. My best position during this run was around halfway up UB2; my worst was around halfway down UB1. I honestly thought my deck bricked more than usual, but 46 sounds like a reasonable sample size, so maybe it's a bricky deck in reality.
All the battles and my result are viewable below. I won 18 and lost 28, with a win rate of 39%. I haven't counted them up, but the most common decks seemed to be Mega Blaziken and Mega Lucario.
List of battles
? means the battle didn't go long enough for me to be sure what kind of deck it was.
- Mega Blaziken L
- Mega Manetric W
- Mega Sceptile L
- Mega Absol / Hydreigon L
- Hitmonchan ex / ? W
- Celebi ex W
- Mega Lucario W
- Mega Blaziken / Entei ex W
- Mega Lucario L
- Hydreigon / ? L
- Mega Lucario L
- Alolan Vulpix / ? L
- Sleep / Darkrai W
- Mega Sceptile W
- Mega Lucario L
- Galarian Obstagoon / Darkrai ex L
- Mega Blaziken L
- Mega Blaziken L
- Mega Sceptile W
- Suicune ex / Greninja L
- Mega Absol / Hydreigon L
- Meowscarda ex L
- Magnemite ? W
- Entei ex ? L
- Mega Blaziken L
- Mega Lucario L
- Galarian Obstagoon / Darkrai ex L
- Charizard / ? W
- Flygon ex L
- Darktina L
- Mega Blaziken L
- Mega Sceptile L
- Entei ex / ? W
- Mewtwo ex / ? W
- Jolteon ex L
- Chien Pao ex / ? W
- Sleep / Darkrai L
- Mega Altaria / Sleep / Darkrai ex L
- Riolu / Falinks ? W
- Mega Charizard Y / Entei ex L
- Mega Lucario / Hitmonchan ex W
- Mega Charizard X L
- Hitmonchan ex / Lucario W
- Mega Lucario / Sleep / Darkrai ex L
- Mega Lucario W
- Hitmonchan ex / ? W
This was my deck:

It does really struggle with megas. Megas generally can one-shot Rapidash or Ninetales on two energy, but they have a high enough HP, even without healing, to survive two or three hits. If the mega is set up on the bench, it's really difficult. On the plus side, water decks aren't that popular right now.
It is possible to play some random deck in Ranked, but you might just end of treading water, and it wasn't that fun, to be honest. I still like the deck and I have fun in random casual battles, but I think I won't be bringing it back to Ranked again.
Mad respect for actually bringing your favourite deck to ranked battles 🫡
I just know my boi Magcargo would get curbstomped out the gate 😭
Looks like Suicune would do very well here. So many fire decks, and Lucario single-handedly got rid of the lightning decks that Suicune is weak against.
Also, interesting that your experience matches this post. Maybe people just enjoy playing fan favourites in general (which is why they are fan favourites 😅)
I'd like try removing Rapidash to make a Ninetales/Arcanine deck, but then I'd probably end up removing Blaine as well and it would be even worse. I'm still going to try it, though.
I'm not sure why Suicune decks aren't more popular right now, although the draw ability can work against it a bit when everyone has copycat.
Mega Blaziken is a decent deck, and I imagine amost everyone has the cards to actually run it, so it's really accessible. If I wanted a more competitive deck, it's probably what I'd use.