Hardest gym leader?

If you did not choose Bulbasaur as a Starter, Misty is a high qualifier! Both of her Pokemon have to move "Recover" making you almost unable to knock out any of her Pokemon. And worse, you NEED to defeat her in order to access the rest of the Gym Leaders (bar Brock, who is actually easy no matter which starter you choose, and Mankey are readily available on Route 22.) She was not that much of a threat in HG/SS.

Sabrina is also hard in FR/LG, because she uses Psychic-types, and the only weakness of Psychic is Bug-types, but there is a distinctive lack of adequate Bug type moves. Not to mention her Alakazam is a monster! With HG/SS giving access to moves like Bug Buzz, she is actually easier in that game.

If you lack a Water-type, Giovanni is a nightmare, I mean, his Pokémon know Earthquake, which can hit even a Digging foe, and even does more damage when the foe is underground. Bad news, I did not choose Blastoise in either FireRed or LeafGreen.

I would have to agree about Whitney being hard to defeat without Fighting-type Pokemon.

Morty is a nightmare unless you have a Noctowl with Insomnia ability. His Gengar is fast, and knows both Hypnosis and Shadow Ball. We're doomed!

Not to mention the final Gym Leader, Clair. Her Kingdra is a monster, having almost no weakness...

If you chose Treecko, Wattson is definitely on the list. Unless you have a Geodude or Graveler on ya, you're ready for a beating.

Flannery is also hard. Her Slugma are easy to defeat, but Torkoal is a whole 'nother story. My suggestion is to use a Rock-type against Flannery. This works in Ruby/Sapphire, but she throws in Numel AND Camerupt into the mix. The best you can do is Azumarill, which can evolve from the Marill found on Route 117. Best use a female though, as Camerupt knows Attract, and is female. Other than that, use Rock-types on the rest of her team.

Next up is Norman, who is a total monster if you lack a Fighting-type Pokemon. Slaking can really take out your Pokémon.

Crasher Wake has a flipping Floatzel, the fastest Pokémon in Generation 4, and can hit hard. His other 2 Pokemon are no problem with their respective weaknesses.

In Diamond and Pearl, Fantina is frustrating, but not as much in Platinum.

Volkner is easy in Diamond and Pearl, but in Platinum, it's a whole 'nother story! He has a Jolteon, the fastest Pokémon in the Sinnoh Pokedex, as well as a Luxray with Ice Fang, and a rather quick Electivire with Fire Punch.

Powerful moves makes Lenora a beast in the classic. However, if you caught a Sawk, she can be handled well with Low Sweep.

Any Trainer with a Zebstrika, especially Gym Leader Elesa. Zebstrika is a super-fast beast. In fact, the only Pokemon in the original Unova Dex (introduced in Unova only), with a higher base Speed than Zebstrika, is Meloetta in Pirouette Form. In Black and White 2, Zebstrika overlevels all the Pokemon in the surrounding areas.

Last but not least, Drayden. During the Spring, Summer and Autumn, Ice type Pokemon are pretty much non existant, making it seem like the game requests you to play during the Winter. In Black and White 2, Ice-type Pokemon are very rare this early on, even in the Winter. For Ice-types, you have Lapras on Village Bridge and Dewgong which evolves from Seel in Seaside Cave, both of which available all year long. Winter only gives off Cubchoo on Route 7, and Spheal (and all of its evolutions) in Undella Bay.