Ah, now here's a fan-favourite level if my memory serves me correctly... Metal Harbor! Why is it a fan favourite? Well... it's a Sonic stage... that's usually enough. XD
Anyways... this stage from Sonic Adventure 2 Battle is quite short and demands a lot of precision if you're looking for an A. There's almost no room to screw up and it's almost easier to just grab all the rings than it is to get 20,000 points. There's only one out-of-the-way ring and it only takes a couple seconds to get.
I wouldn't feel content getting A's on it that way though, so I do it the old fashioned way! Thus, here's my run of Metal Harbor... enjoy!