For the highest quality reproduction carts you can't go past Charlie-802.
Charlie 802 is a gold plated, 4mbyte Flashable cart available in MBC 1/2/3 or 5. Instead of the traditional Battery backed SRAM, Charlie uses FRAM which needs no battery and will never loose its save file.
Currently does not support RTC so in-game RTC events will cease to occur.
Tested with Pokemon Red/Green/Yellow/Blue/Gold/Silver, DKC and various other titles.
Fits perfect in a genuine nintendo cart shell and can be purchased blank or pre-programmed.