For me, it was definitely Tate & Liza.
Heonn back then was my favorite ruby and still is. (I WANT A REMAKE xP)
I had a copy of Emerald and Sapphire. In my sapphire I start with a Torchic which of course ended up as a Blaziken. Since Tate & Liza had a Lunatone and Solrock, it was a real pain trying to beat them. Especially since the Defense, Speed, and Special Defense for both Lunatone and Solrock is freaking annoying it was incredibly hard for a Fire-Type. Immediately I thought of the DynamicPunch but, they're stupid PSYCHIC/Rock-types so that made it not too effective. Thus, I had to mainly depend on my Ludicolo and his "Surf".
(NOTE: This was a little while back slightly before GameStop stopped selling anything that had to do with GameBoys)
In Emerald though, it was much easier because I had found out about "Soft Resetting" and I really didn't want to start my Sapphire over and I really wanted an Emerald version so I thought, "What the hell, let's buy an Emerald copy!" So I bought a copy and did about 3,500 soft resets. I had been taking tallies on a piece of paper with me, but I gave up slightly after 3,500 so I did it a couple dozen more times and I accidentally clicked Mudkip, I yelped: "Crap!" but then I had a huge spaz attack because it was a shiny one, so of course I kept it. When I got to Tate & Liza it was an easy win for Ludicolo and Swampert.