On December 28th 2014, I got my 8th win of at least 1000x bet, when I won 3467x bet on Santa's Wild Ride at 32Red.
On 11th May at Nostalgia Casino, I got my 9th. And my first of 2015.
Pic 1 - My first 4-reel WD. 1575x bet. Which makes it my 3rd biggest win in x bet terms.
1. 3467x bet - Santa's Wild Ride, December 28th 2014 @ 32Red.
2. 1903.5x bet - The Dark Knight, July 2014 @ Yukon Gold Casino.
3. 1575x bet - Immortal Romance, May 11th 2015 @ Nostalgia Casino
In terms of cash, it's my 4th biggest win.
1. £1,200+* - Iron Man, Casino Tropez, September 2013. (£1 bet)
2. £1,142.10 - The Dark Knight, Yukon Gold, July 2014. (60p bet)
3. £1,040.14 - Santa's Wild Rice, 32 Red, December 2014. (30p bet)
4. £945 - Immortal Romance, Nostalgia Casino, May 2015. (60p bet)
* Memory of exact amount won/x bet win is sketchy. Maybe I don't remember it so well because I never got to cash it out
due to the ball-busting WRs on that particular deposit (it was in my pre-CM days when I was a bit more erm..."naive")

But it was definitely at least £1,200 and that's enough to make it my biggest "win".
And it takes the list of games where I've had a 1000x bet win to 8. In chronological order....
Sultan's Gold, Iron Man, I'm A Celeb, Dark Knight (twice), Playboy, Couch Potato, Santa's Wild Ride, Immortal Romance.