For basic flash cart functionality I'd recommend the Super UFO pro 8. Games like TMNT 4, International Super Star Soccer, Offroad 2, Street Fighter, Donkey Kong, Megaman 7 etc work great. Once you replace the battery (and know your way around the sram method) you can play Super Metroid, Zelda, etc.
For games with special chips or too large (ie lots of "mega power"), I either get the cart (looking at you Yossy Island) or fire up le Wii. No flash cart is capable of playing everything anyways.