IMO, (and forget financial fair play etc, we'll all cling to whatever ****ty stick is offered if it will save us drowning and Derby, Forest and Leicester have all been there):
Derby's and Forest's achievements are equal, and equally massive. I personally class any league win over any cup win and I could consider a single league win as a 'fluke' but not a double league win (Derby) or a double Cup win (Forest). I'm talking the glory years here obviously, if I included the Brian Clough Trophy there would be no contest.
Leicester are some way behind with a single league win, let's see if it is a fluke or the beginning of a Leicester legacy. I haven't even got an opinion on that
