1. Cilan- 1/5 (if you use a Pansear), 2/5 (if you don't use the Pansear) The gym battle is more difficult if you don't go into the Dreamyard first to pick up one of the monkeys. That said and done, Cilan's pokemon would usually waste their first turn with Work Up. As Flying-type pokemon are not available at this point of the game, you must rely on the pokemon you trained thus far. Pansage should 2HKO most of your pokemon (other than your starter). Just attack it. Cilan would sometimes use a Potion on his Pansage.

2- Lenora- 1/5 (if you use a Fighting-type [Sawk, Timburr, Throh]), 3/5 (if you don't have one)
Lenora is just like Whitney in HGSS. She's only going to be a real problem for you if you don't use any pokemon that have weaknesses against her. At this point in the game, her pokemon are incredibly powerful second-stage pokemon with decent speed and attack power. I used a Sawk for my runthrough of Black, so I managed to sweep her entire team only because of the STAB, weaknesses, and higher ATK power.

It will be much more difficult if you use a Pignite or another Fighting-type pokemon, which are not as blessed in high Attack or Speed this early in the game. That Miruhog is able to use Watchog to Hypnotize your team, and if you managed to defeat the Herdier before it, Retaliate will often OHKO your team.

3. Burgh- 2/5, 3/5 (if you are stuck with no Fire-type or an underleveled Pansear)
Bug-type gym leaders should be very easy, correct? For the most part, the only trouble you'll run into is that Whirlipede. It has high Defenses so most of your physical attacks won't even 2HKO it. It has access to Poison Tail, which often poisons, and Protect (to protect itself numerous times). Dwebble is somewhat easier, as it is cursed with the horrible Bug/Rock typing. Ignore the Sturdy for now, and you'll notice Burgh would almost never heal it back up to full health.

His Leavanny could be a difficult opponent if you don't have any Fire or Flying types, or if your Flying type was knocked out already and you didn't have any Revives on you... In this case, as a fully evolved pokemon, Leavanny has somewhat high ATK and a respectable Speed stat. It has been blessed with Razor Leaf, which often provides a critical hit ratio, and Protect (so fire-type attacks normally wouldn't be able to defeat it on the first turn). My Pansear in particular was underleveled (as I planned to catch a Darumaka later on). It didn't even know Flame Burst yet (only that horrible Incinerate). Because most of my pokemon weren't effective against Grass-types, I lost the first time, packed a few more Revives, and defeated them with my Tranquill.

4. Elesa- 3/5
Ground-type pokemon would normally be the cakewalk to defeat an Electric-type gym leader, correct? Well... As far as Ground-type moves, Elesa uses two Emolga that completely immune to them (unless you use Smack Down or something like that...) I used a Sandile equipped with Rock Tomb and it only inflicted half of its normal damage. Even so, Emolga completely outsped my Sandile at the start. The Emolga usually have STAB Aerial Aces and Volt Changes to switch around with each other. If not, they would go around for the defense with Double Team.

Her Zebstrika is another issue, although it is not as difficult. For the most part, you will have to resort using a Ground-type. Zebstrika has high ATK and Speed and could almost OHKO most of your pokemon (this is coming from a guy who is using primarily water and flying-types on his team). I had to revive my Sandile, give it some Super Potions, and let it just spam Dig until it defeated it.

Boldore is a recommended choice to resist the Emolga because of its high Defenses. However, since Boldore usually aren't available until later on in the game (and in great numbers too), most trainers would not see the merit in using one this early.

5. Clay- 1/5 (2/5 if you let Excadrill use Hone Claws multiple times)O
Since this game gives you a lot of Water, Grass, and Fighting-type pokemon, this is perhaps one of the easiest gyms in the game. A Swadloon I caught in Lostlorn Forest was enough to defeat Burgh's Palpitoad and Krokorok. His Excadrill could be an issue if you allow it to use Hone Claws more than enough. I suggest getting a bulky Water-type pokmemon like Dewott/Samurott or Palpitoad as a counter. That Excadrill could be a real problem if you let it keep using Hone Claws...

6. Skyla- 1/5
Alright, let me just say this: Easiest gym leader EVER. I was using a Tynamo I caught in the Chargestone Cave during the entire playthrough, it was "cursed" with four moves (Tackle, Thunder Wave, Spark, Charge Beam) until it evolves at LV 39, and it STILL defeated every trainer and the gym leader here! Skyla is very easy if you pack an Electric or an Ice-type (such as Vanillite), though that might not be recommended against Swanna.

That said and done, her Unfezant would sometimes Detect your pokemon's electric-type attacks, but overall that is more of an inconvenience than a problem.

7. Brycen- 1/5
So many Fire, Fighting, Steel, and Rock-type pokemon in the game... I am surprised this guy is number #7, to be honest. I was using a Meinfoo I caught in the Dragonspiral Tower for this gym playthrough. It was incredibly easy. Meinfoo is my recommended Fighting-type here over the slow and bulky Conkeldurr. Conkeldurr have very low Special Defense and Frost Breath provides a Critical Hit all the time. Meinfoo has higher Speed and a respectable Attack stat as an unevolved pokemon. You can catch it at Level 30, and at that time, it is already blessed with the likes of Force Palm (Drain Punch at LV 33, Jump Kick at LV 37). I defeated Brycen's entire team with just that pokemon.

Sometimes, his Cryogonal and Vanillish would use Acid Armor to boost their defenses, but when you are using a Special sweeper, it will not be any problem for you at all.

His Beartic is not a problem, either. Sure it is Bulky and has access to a lot of Water-type attacks and Swagger, but Meinfoo's Drain Punch sure packs a punch.

8- Drayden- 2/5 (if prepared), 3/5 (once his Haxorus gets going with Swords Dance or Dragon Tail)
Dragon-type pokemon are a novelty in the games. It houses some of the most common types as resistances, are known for either excelling in Special Attack or Attack, and a somewhat respectable Speed stat.

Yes, this would be the case for Drayden as well. I used a Fraxture for my walkthrough, and because it had a Speed boosted nature, it was able to outsped all other Dragon-types in the gym. It OHKOs the opponent's Fraxture and Druddigon. Once Haxorus gets going, however, you are somewhat intimidated. Don't let a fully evolved pokemon get you down; with the amount of training you had to go through this far, it shouldn't be too much of a problem for you.

My Archeops completely swept this thing.

This might be the easiest gym bunch ever, but let me assure you that for those who do not have as much preparation for these battles, it will be very difficult at first. Arti and Elesa were all challenges for my Water and Flying types, and I didn't really have access to any powerful Fire or Ground-types yet in the game.

Later on in the game you can catch more pokemon with specialized types and stronger stats. This is perhaps the reason why the later gym leaders are much easier than the first bunch.