Ace Trainers Dennis and Maya. It must have taken over 30 minutes to wallbreak that (can't remember which mons or version that was) the first time I faced it, when hyper offense was the only playstyle I knew. In the other games, it was easy.
Cyrus in Celestic Town. I never bothered to train up a Ponyta or Machop, and I started with Turtwig.
Are you talking about the ones in Diamond and Pearl on Route 215? With the Gyarados? I actually didn't pick up Diamond for my first game and knew what I was doing when I hit them.
My first game was Platinum, though. Probably the hardest game in existence, at least for an inexperienced player. I'll just list some of the encounters you have to go through:
-Roark: I breezed through this first time since I had Prinplup, but I recently returned to the game with Chimchar and holy shit Cranidos murdered my Monferno (me having a Hasty nature didn't exactly help) with a crit Headbutt. My first time blacking out on Gym 1. >.<
-Gardenia: Thank god Roserade doesn't have Technician in Gen IV because otherwise this would be impossible. The first time I played through I blacked out ten times here. And yes, I had a lv. 25 Staravia. Turtwig has Reflect, Cherrim has Leech Seed, and Roserade has that giant Special Attack.
-Fantina: Mismagius tho. Duskull with its Will-O-Wisp doesn't exactly help. Not to mention that 2 Magical Leafs+2 hit self in confusion=dead Staravia.
-Dennis and Maya nearly killed me the first time, even without their Gyarados. Drifblim this early in the game is just screwed up.
-Maylene: Protip: Don't evolve your Prinplup and expect to beat this gym.
At this point I got myself a Tangrowth because it looked cool, so it basically soloed the next part of the game. Except for one little thing. Cyrus couldn't even touch me.
-Ace Trainers Zac and Jen. Holy shit. I didn't have Defog, so basically I had to deal with permanent -2 Evasion. And if you didn't know these guys use lv. 36 Gyarados+lv. 36 Raichu in Platinum. Gyarados has Intimidate. Basically I just stalled these guys out with Tangrowth because I had no idea how the fuck to kill them.
-Candice: Abomasnow, followed by Froslass. Froslass even has Double Team, meaning if you miss once, you're never gonna hit again. My only ---% move was Staraptor's Aerial Ace. And, well...Staraptor.
B/W has some hard NPCs too. Striaton Gym is...Striaton Gym. But the one that probably takes the cake here is the Battle Girl in Twist Mountain with a BULK UP THROH. That single Pokemon killed two of my team members in my latest Nuzlocke of this game (although Serperior died because Leaf Tornado missed). There's a Black Belt later on with a Bulk Up Sawk but it's not as bulky.
Nothing beats Juan's pure trolling though. Except maybe Winona's Altaria, if you were looking to have a clean sweep with your Manectric/Electrode or something.