There have been plenty of wtf broken characters in my time playing lol. Funny stuff I remember off the top of my head that I haven't seen mentioned here:
Heimer seige- His grenades hitting towers made him so damn annoying.
Original Jax with 100% dodge. Probably should of just surrendered when he got that point, shit was broke.
Locket meta was also pretty dumb, with gragas and udyr going crazy.
And on a sad, personal note. I really miss old AP yi. He never was op, but he sure as fuck was fun, and I played SOOOOOOOOOO much of him.
Also I don't know what game you guys were playing. Meta was not ADCs mid when vayne came out. She was pretty damn strong, and broke, but she didn't auto win lanes in my experience. The most hillariously dumb champ release I ever remember was irelia. She was fine/a bit OP on release, but the spotlight was SOOO awful. So many people tried to do the same shit when it didn't even work in the video and jungle with her, and they sucked just as much as phreak did. Every irelia ended up being a hardcore feeder pretty much.