One with an idiot-proof instruction book (and an easy to use bobbin, if possible).
I'm only ever going to use it to sew straight lines (curtains, cushion covers, duvet covers), so I don't need any fancy stitches, but if it can cope with thicker materials that would be good.
I'm currently muddling through on my mother's ancient Jones one, and it is rather temperamental.
If it can be bought online, so much the better. I went to look at them in John Lewis this morning and DS screamed and screamed and screamed. We gave up and came home.