I use Asda online using codes from here.Just go to the last page for the current codes .Usually just free delivery (I never have had to pay for delivery using these codes).Lots of offers/3 for 2 on the site.
I like Asda because they substitute with a higher value product if your ordered one is out of stock (e.g. tester £2 bottle substituted for the full £6 bottle) and you don't pay the difference.Also very good at refunding for any damaged/near date items.I just go into Waitrose for the odd bits I like from there.Only thing I like Tesco for is their clubcard points although their item search is a bit more accurate than Asda.It takes a while to use any shop online for the 1st time but they all seem to have the facilty to save your recent products bought which makes it alot easier second time round.
I've tried them all and find Asda the best value although i think alot depends on the quality of service from your local store.
Just saw you asked about delivery.Asda only do 2hr slots (not 1 hr like Ocado) but you can usually get a slot a couple of days ahead.
Just done my shop for Monday morning.Ocado/Waitrose are more convenient therefore but i don't pay for delivery with ASda anyway so it suits me.Tesco slots seem to book much further ahead (near me anyway)- which I find a pain if I don't order promptly.
Remember to book your just before Christmas delivery slot as soon as they are released.It is great to get a delivery when everyone else seems to be braving the bedlam of the supermarket.
Ocado/Waitrose do lovely different coloured bags for freezer/fresh/cupboard to make putting away easier but were more unhelpful when I complained about damaged goods.
I think most people stick with the site/supermarket they are used to anyway.