Even though I chose Sabrina I have to go with what a number of people are saying and say that most of them are actually pretty easy. Sabrina in my opinion is just generally the toughest in a playthrough.
Brock, unless you chose Charmander, he's a wimp. If you did, train up a mankey
Misty, Surprised people actually have problems with her, once again unless you picked Charmander in which case Oddish, or Pikachu can fulfill the same role as Bulbasaur, Wartortle can bite Starmie with taking little damage from it's attacks, and the basic strategy and easiest is just to stun it then overwhelm it's recover all which Pikachu, Ivysaur, or Oddish can do.
LT surge, oh look how cute, a Diglett, wait, how the f*** did it take down my Raichu!? LT surge, I'm disapointed
4th (forget her name) NPCs don't have any clue how to utilize grass types, it's too easy any way you look at it
Koga, What broken team in FR/LG isn't complete without a psychic type Pokemon!? =D
Sabrina, ummmm, uhhhhh, the only easy way is to outrun her Pokemon with a high physical attack Pokemon (which doesn't help if you're underleveled for it) Or if you have Blastoise, it's still quite powerful with a bite, or Lapras can survive one attack from Alakazam, just perish song
Blain, one word, Lapras
Giovani, one word, Lapras
Sorry for spelling, I don't feel like proof reading xP