If you can afford it, get a Super Nintendo N64 GameCube RAD2X HDMI cable. If you want something cheaper, get the Hyperkin 3-In-1 HDTV Cable for GameCube/ N64/ Super NES. You get what you pay for, but even if all you could afford is the Hyperkin cable you'll be fine. You can always upgrade later.
If OP is using an HDTV, I recommend going a step further and buying either the Oneme N64 To HDMI converter, the Laboen N64 To HDMI converter, or the Hyperkin HDTV cable... All three are relatively cheap.
If you want even better quality, go for the EON Super 64 adapter, which is the one I have and works great. Or you can go all-out and mod your console with an UltraHDMI mod.
Here is the one I got. Recommended in a different sub. Ill tell you if it is better but i am more focused on the PS2/Gamecube era even though i plan on using an n64 eventually