Well upto now I faced upto Luna and just clearing things before Agate Gate. I am a recent player for this game so i will rank all these 9 gym leaders based on my team on how i faced them.
Easiest:
Luna : may be because of my team it's way more easier to counter her. I didn't even change her filed My team is staced with Heracross, Bewear and Primarina cutting Luna easily down.
Shade: It's obvious the most mysterious character is actually pretty easiest gym till now. Though his gengar(corey) has hypnosis, just avoiding it's accuracy is enough. By that time i have a phantump, and a stunky to deal with shade better.
Serra: Special team and serra goes brr.... Seriously with all the special attackers with me by that time i took down serra with ease. I have a simisear who whacked through half of her team, primarina who supported and bulked in this process and a swalot with toxic that took hits and stalled everything against her.
Moderate:
Julia: may be because of my starter(popplio) i faced a bit against julia. I need to take care of her speed especially not going to set her oricorio a tailwind. But apart from that the weak charge beams didn't do me much yet i need to tactically think of her so yeah julia is moderate for me.
Randomus: The trickster is either easy to break or a menance there is no inbetween for him. I faced him in 3 play throughs the first two where i got demolished and the third where i demolished him. The main thing is which position pieces you need to keep your pokemon. My palossand, trevenant and queen stunktank did a lot of help in my 3rd playthrough. So yeah basically good positions clears randomus. else he is a menance.
Shelly: So, if it's not for primarina i may have suffered at her field and bugs, but thanks to primarina and it's sparkling aria. I didn't let her anorith tear against me. With help of my bewear's thunder punch i stopped the grass hazard pokemon yanmega or araquanid. Shelly i played 3 playthroughs because understanding her field is really tough but as i progress my team actually stepped over this hazard with ease luckily.
Aya: So, basically all the three double battlers till now are moderate for my team. For aya I put a klang in my team to move against her poison weaknesses. Primarina to eliminate the ground move pokemon like nidoking and talonflame for the extra speed. So basically i stacked a team against her (with swalot to take hits). and slowly cleared aya but honorable mention if not for my timely switches aya made me struggle for sure.
Hard:
Florina: Damn that ferroseed and cradily. Florina being a pokemon leader with a type of many weaknesses managed me to bite my nails. Her field is a problem for me making things difficult for flying and poison typings. The fire type pansear didn't work a lot and taken down easily so i really struggled to get past florinia. I need to make huge changes from my julia's team to florinia's team and need to do lot of mind map on assuming when that damn cradily would come out.
Noel: Absolute menance. The magic guard clefable is a nightmare for sure and his remaining normal types are not to downgrade. By this time i don't have a heracross, so my fighting warrior was bewear. and I used palossand and trevenant to miss his attacks but yet i struggled a lot to go through his team, may be because fo movepool. I struggled like hell and needed to play nearly 6 playthroughs to beat him. Noel went straight up demolishing me with his pokemon. So he gave me the hardest time till date.
For corey i would say he is easy for me and kiki gave a moderate time because i thought of a flying option against her and took 3 flying types which is actually dumb to do with her field but i slowly cracked it with help of my primarina and palossand.
So now i am at agate gate and most of the pokemon i use are as mentioned above. I generally rotate between them (regently caught and aron and made it aggron so include aggron too). Hence you can give suggestions on which other gym leaders may give hard time to me in future with my team, and what's better to catch before i reach agate city (i think i need a psychic type for sure but which one should i catch you can suggest me for sure).
