Latias in HeartGold a few years ago. In HeartGold, Latias is a roaming Pokémon so of course it takes a lot of time and grinding to find her and when I'd tried to use a move like "Mean Look" to ensure she didn't get away she'd get the first move and flee before I could trap her. So I followed her all over Kanto until eventually I just decided to keep running back and fourth between Lavender Town and Route 8 until she came to that specific route and once she did I saved my game and just ran around that route until I found her and when I did I made sure I had a fast Pokémon that be sure to move faster than her and trap her with mean look so she wouldn't run. Once I did, it was on and I battled her like Rayquaza vs. Deoxys. However it took a bunch of tries since she'd K.O. herself with struggle or I'd accidently K.O. her myself with critical hits (Seriously, whenever I DON'T want criticals I wind up getting them on Pokémon I'm trying to catch while my opponent gets like 1000 in an actual battle). So I kept restarting and repeating the process until I FINALLY caught her and appropriately named her "Big Mona" and since then she's become one of my strongest Pokémon in HeartGold earning a spot on my team of dragons until I replaced her with Salamence. Although I prefer Latios over Latias, it was still a good catch.
Best part about catching Latias, I caught her with a standard Pokéball like I do all of my other legendaries. Yeah that's right, I'm a REAL man. Great Balls, Ultra Balls, Masterballs those are for dorks and noobs. REAL MEN catch their Pokémon with regular Pokéballs. I've been regularly catching my legendaries with regular Pokéballs since 05.