There is a thread on OM
http://www.outdoorsmagic.com/forum/gear/what-dry-bags/65259.html and the whole subject comes up again and again on this and other sites.
Basically, all you want is a variety of sized bags, which are reasonably tough and waterproof, and which dont cost the earth. There tends to be two types (at least for hiking) - basically, most drysacs are taffeta/sil nylon, or the slightly thicker ones with a rubber like inner coating (I've got some like that from Blacks and via TKMaxx from Trekmates). Exped is the major name for the sil nylon ones, but there are loads of others - Sea to Summit, Alpkit Airloks, Podsac, etc. I hear decent things about karrimor ones, and they are a decent price. In fact drybags are a good way to twig if a shop is overcharging - its a basic design, using the the same materials - if you are paying way more than the average, then they gouging on the little things. And since pretty much general hiking store carries Exped, its pretty much the Mars bar of the hiking world - its just standard.
I tend to buy mine when they are on deal, or are just well priced. Ones I like are Alpkit's Airloks, which are very well priced, and their Xtras
https://www.alpkit.com/featured/dry-bags are fantastic value - really tough (closer to the rubber lined ones) with attachment points. I've got some Sea to Summit from TKMaxx, which I havn't tried out, plus some other brands, like super cheap ones from Wilkinsons. Podsac are having a big clearance of their stuff (they've been bought by another company)
http://www.podsacs.com/c/q/bags-covers-liners - so grab them while you can. I bought mine from Blacks (they had a stupid online deal), but the Podsac website still has some good deals, especially for the packliners. They charge £3.95 delivery on order under £50, but if your looking for drybags and a new pack (at a very good price), fill your boots.
I hear good things about Lomo, but I've got probably more drybags than I really need (although the Podsac packliner would be useful...), so I've not tried them out.
Ones to avoid - really cheapie ones from Chinese online stores, and stuff from Eurohike - rubbish. Also surplus stuff (its generally knackered) - I bought a cheap US packliner from my local surplus place, and it was full of holes - a waste of time.
Ortlieb are really heavy - bombproof and great if you kayak, but I would want to carry one - they do lighter bags as well, but they are not cheap.
For a decent product at good prices, I'd try Podsac while you can, and Alpkit for a good general deal. The other brands are fine as well, but generally cost a little more.