From the Macdermott files, a the largest loco ever to run in the UK -the LNER Garratt.
Building a model of this has always been high on my wish list. I've seen both 4 and 7mm versions and they are massive. The square bunker always looks better than the rotating version fitted to the LMS machines though, the same reason I prefer these in their unrebuilt form.
Anyway, if you have loads of time and a fair amount of cash to spare, here are some links to kits:
7mm scale - Gladiator Models.
4mm scale - DJH Models