I used to have a hard time going against Tate and Liza. Well, that was only because when I was playing Pokémon Ruby for the first time, I only relied on my Blaziken to do all of the fighting. Granted, it wasn't too bad, but.. the type disadvantages really hurt. Tate and Liza made me realize this, since her Psychic pokemon was really tricky, ugh. Oh, but I do really like them as Gym Leaders! I liked that idea on having co Gym Leaders, and you'd have to beat them in a double battle. I wish more gym leaders were like those two!
Another Gym Leader that I liked on Pokémon R/S/E was Flannery. She's pretty awesome, and her character design was amazing! Love her hair, aaah. She was actually pretty fun to fight against too, because I love fighting fire pokemon in general to try and match their power, and her pokemon didn't disappoint! Her Torkoal was definitely a challenge to fight against because it had great defense and her Iron Defense helped that out, but, her spamming Overheat preeeetty much was her downfall. XD
Hm, on FireRed and LeafGreen, on the other hand, I didn't really have any trouble fighing the gym leaders because, my experience of fighting them on Pokémon Blue and Pokémon Yellow carried on over there, but if anything, I'd say Misty was a little tricky when I was trying out Charmander. Those Water attacks aren't a joke, especially if you were using a Charmeleon. XD But still, Oddish's and Bellsprouts were around, and I've also used Pikachu, which was a big help.
Ah, but I have a whole bunch of favourite Gym Leaders on Pokémon LeafGreen/FireRed. The first is Erica, because I really adore her, and her traditional Japanese-woman design was really sweet, and awesomeeee. She wasn't too hard to beat, especially since I've had a Pigdeotto and a Charizard at that time. Plus her Gym design was always a pleasure to visit. I also loved Sabrina, since I really liked her FR/LG design more than her HeartGold/SoulSilver design because, it made her feel more fierce, hehe. Hm, on terms of fighting her, yeah, her pokemon were pretty strong if your own pokemon weren't that strong themselves, but, it wasn't too bad! I've used a Snorlax on that battle, and it's Special Defense was pretty impressive! Amnesia really helps. C: