Panasonic if your going plasma, don't really rate their lcds.
Plasmas are better for faster moving tv, but tbh I don't think the average consumer will notice.
Make sure it's 1080p resolution, 720p IMO is a wast of cash these days. If you keep an eye on your electricity bill, you will notice it rise with a plasma, it might even drop with a new gen LCD.
IMO I wouldn't pay extra for 3d, but if the set comes with it then thats ok.
Smart gives you Iplayer and such, no use to me because my tv is connected to a pc, but if you feel those features will be helpful make sure the tv has built in wifi.
If I was in the market for a tv again I'd be going Samsung plasma or LCD, purely based on value for money.
John Lewis is good as they offer 5years warranty on everything, they will also price match so shop around then get them to match it.
When you find a model number jump on avforums.co.uk and read the owners there'd of that tv, they will highlight issues and drawbacks, avforums may have reviewed the tv too.