During the FuncoLand days I looked through their flyer and saw "Raiden" as a game listed for I believe Super Nintendo. I didn't realize that it was an overhead shooter or whatever it was until I asked someone if they made a Mortal Kombat game centered around him.
Since MKT had both MK2 and MK1 Raiden, maybe we could assign one of them the I and one the Y. Although which one gets which is debatable too, since they have both been spelled both ways.
I think the end goal of all versions of 2D MK is the arcade version. It's what the home ports are based on, and MKT is good because it attempts to reproduce UMK3 as best as possible.
So I say Raiden. It's his original name. It's more accurate for what he is.