From a randomly buying cheap consoles at the flea market and seeing if it will just plug and play standpoint, I've seemed to have had the least amount of trouble with sega genesis systems. If just speaking of Sega; the Sega CD and 32X is a different story. I've done minor DIY repairs to those. Saturns are usually fine sans the dead memory battery. Dreamcast I've just recently been noticing controller drift and other controller issues lately.