Hardest Unova Gym Leader

[Hircine]

I'm going to have to say it's a tie between the first and second gym leaders respectively. The first only because I always start with Oshawott and Pansage's Vine Whip and Work Up combo is killer against a water-type; Lenora because she was obnoxious in general and Herdier / Watchog tend to be very tough early on to beat, since your Pokemon are rarely evolved.

[Ferrin]

lenora's gym was hard. retaliating wachog with hypnosis... OF DOOM

[Seredin]

I'd say Cilan/Chili/Cress because it is quite difficult to win, considering that they use the better type, and I don't use normal types often, so I usaully have to Evolve then fight, or have a bit of luck! XD

[PowerMiner]

Burgh. Definitely Burgh. I had to try a thousand times to beat him... Harder than any other gym leader for me. I wish I could replay Unova, but I have like 600 in my pokedex there, so... Maybe I'll just reset and have the same name after I finish the national dex and trade the pokemon (And ALL my other good pokemon) over to my White 2.. Except I don't think I'd have enough, so... I'll just have to wait until Generation 6...

[Darkazure]

Lenora and her watchog were the bane of my pokemon career. Took me a DAY(more then any leader) and 79 battles later to actually beat her.

[BIG MIKE]

Lenora and Burgh

[Mystical Jackal]

It was Lenora. Her Watchog's Hypnosis seems to never miss against my Pokémons

[ansem the wise]

dreyden, i had to leave and catch a cubchoo to beat him.

[pokemonsquared]

Cheren, Arceus that stupid lilipup

[Plus side down side]

Elesa and her stupid volt-switching Emolga got the better of me. My team wasn't equipped very well for electric types, but then adding flying into the mix made it worse.

[Kutie Pie]

Oh good Lord, Lenora was hard for me, if only because I didn't really have the best Pokémon on me, and none of them were faster than her Watchog for a while. Elesa was annoying with the stupid Volt Switch, but I managed to beat her in one try, and Clay wasn't too difficult once I leveled up my Pokémon some. Other than that, the gym leaders weren't that bad.

[Crimson Penguin]

I think Lenora was the only Gym Leader who beat me on my first playthrough. Stupid Watchog... Didn't help that my Pokemon were a bit underleveled and could only take two or three hits before fainting.

That said, I didn't find the Unova leaders too difficult as a whole.

[DizzyCobraMew]

For me on black it was always drayden, white 2 it is probably Rosie as you don't get counters for her team wil later in the game

[Dragonquester]

I hated Lenora because of the level gap. The gym goes from level 12 to level 25. I hated it.

[Luxvan]

Lenora and Elesa easily.
Other gym leaders were laughable, honestly.

[EspNeon]

For me it was Elesa.It took me 4 different attempts before I found a stratergy to beat her. It was the Emolga's and Volt Switch that caused a lot of trouble since they are not only fast but could keep alternating meaning it took longer before I KO'ed them. Luckily my Blitzle (which I hated) had Lightningrod so for every Electric attack used, my Special Attack went up. Then I spammed Flame Charge to out speed them.I out trolled the troll.

Cilan/Cress/Chili were hard to defeat for a variety of reasons. Firstly there are no Fighting types to quickly dispose of Lillipup, so the longer it is in battle the more it can use Work Up and become deadly. Then of course the Elemental Monkey(s) respectively have work up and Vine Whip/Water Gun/Incinerate.

Lenora and her Watchog/Hypnosis/Retaliate are a very lethal combination.

Burgh is quite an underestimated powerhouse in his own respect. His Dwebble is part Rock so it can take out any Fire and Flying types with it's Smackdown (both of which are SuperEffective against bugs). Then his Leavanny packs quite a punch.

Elesa and her Emolga's are very fast and the continuous swapping makes it much more difficult to focus on taking just one of them down. Zebstrika is strong and well rounded, give an added pinch of speed from Flame Charge and it can become deadly. Especially if it is a long match.

Clay's excessive use of Bulldoze becomes a gradual problem since it lowers your Pokemon's speed. If you stick to using the same Pokemon without swtiching by the time Excadrill is up for battle you will easily be slower which is not a good sign since its got a decent attack.

Skyla isn't much of a problem permitting you can bring a strong electric type to sweep her team. Swanna's Aqua Ring can become a pain but otherwise she's a pretty easy sweep.

Brycen's biggest threat is Beartic with Swagger/Brine. Cryogonal and Reflect can be a pain if you're using a Fighting Type but Brick Break (available in Iccirus) will soon take care of that.

Drayden/Iris tend to rely on Dragon Tail and although it strikes last due to low priority it can soon be a problem when it rotates your team members around and throws your strategy off of balance.

In my opinion, the first 5 gym leaders are the biggest threats. The other 3 not so much. I'd rank them like this:
Elesa -> Lenora -> Clay -> Cilan/Chili/Cress -> Burgh -> Drayden -> Brycen -> Skyla

[OneyNG]

None of them are hard as long as you train your pokemon.

Brycen was hard for me. He was a little ash-hole. (See what I did there? I don't.)

[Luthor]

i didn't have any real troble with any of them but i voted for iris because that was the gym that i was the least well equiped for (only had 1 dragon type which was weaker than the gyms and no ice types) .Still won it first time though.

[Orithan]

For me, it was Lenora. While I managed to beat every gym leader on my first attempt, the fact that I saw how strong the pokemon the other trainers had in the gym (and how powerful Herdier was) was enough to make me grind for nearly a whole day to get my pokemon to a level where they would make her manageable. The fact that I didn't want to use Throh or Sawk didn't help my cause.

[Super Llama]

Well, this discussion is meant for the BW1 but I'd like to say that Marlon's Jellicent was quite the pain for me.