One of your biggest concerns might be inputs - you need something that will be able to hook up to all those old systems. Some output only composite to go through the tuner, some will output composite or s-video, and some proper RGB.
AFAIK:
NES - RF out only
SNES - multi-av out, pretty sure s-video is the best quality it does as stock, but you can relatively easily mod the consoles to get RGB.
N64 - Composite is the best if I remember rightly, no way of modding to get better unless you buy a US/French console.
Mega Drive - can't remember exactly, but there are differences between MD1 and MD2.
PS1 - pretty sure does RGB