Aside from the obvious choice of Mechagodzilla, most of the things I'd pick would be from Power Rangers.
I was always fond of the Q-Rex (yes, that stands for Quantumsuarus, because fuck you) from Power Rangers: Time Force. I loved the three or so recycled clips from the original series of just the Tyrannosaurus fighting, and this was basically a souped up version that didn't look stupid. Instead of having this big dumb looking boxhead, it was sleep and viscous looking, and it didn't try to swing around it's little arms, it would just duck down and streak into you, latch on and try to thrash you around until you were on the ground, or whatever was in it's mouth gets torn the hell off. Also helps it could turn into a biped with a tail and blow your ass up with a revolving missile launcher arm and twin shoulder mounted energy beams.
Now, if you want to see how NOT to design a giant mecha robot, look to RPMs Ultrazord. It's like someone was blindfolded, covered in glue, spun in circles and then sent running through a toy store.