The only griddle that comes any-where near to being even across the entire surface is the accu-temp griddle.
It uses a sealed steam chamber to heat the griddle from the underside.
Even using this technology (only one I know of) they still only claim +/- 3F for accuracy.
That is way way way better than anyone else out there which will commonly say that 50F is 'good enough' and in reality it commonly is.
The top is stainless steel however - so be prepared to use lots of non-stick spray or oil on foods that have low fat content as it doesn't really develop a patina. (season)
They can be polished quite easily however and then they become pretty slick but you have to be careful with the surface ... no banging of spatula's or turners etc.
One cool side benifit from the steam heating the steel surface is that the temp recovery is incredibly fast. Seriously fast... I burnt quite a few things before I figured that one out. (should have read the manual as it does talk about this!)
Basically when you put food down the temp of the steel drops causing condensation inside the 'sealed chamber' this condensation drips back into the bottom and is replaced by 'hot-steam' and the cycle repeats, very fast.
I call it a 'quick-griddle' as even set to 350 it cooks things quite a bit faster than normal cast-iron griddle - think of it as a 'convection-griddle' and treat it like a convection oven. Set 25 degrees lower or learn to utilise the heat recovery.
You can actually flip burgers etc. onto the same spot they started and not see any real difference in browning speed.
I like them... wish my current place had one! (hint hint)