>>256208630 liter folding top dry sacks. 30L to 35L is a good size for pretty much anything. Not too big that it can easily rip or be loaded too heavy.
Liners break too easily at the most inconvenient times. But if you are backpacking near supermarkets and such they are easily replaced, and useful for weekend trips.
Ultra-sil, 20D, 30D, is not durable enough. I had good experience with Snugpack and Tatonka dry sacks. They seem to last a long time.
Go double on your sleeping bag. When it rains heavily for a few days one waterproof barrier is not enough.
For my long trips I use 3 bags:
1 for the tent
1 for food
1 for sleeping bag and clothes (extra dry bag inside for the sleeping bag)
zip lock bags for toilet paper, toiletries, double zipper bags for electronics.
extra equipment that you have to use and keep dry frequently like phone, down jacket, gloves etc. need dry bags aswell because zipper bags break too quickly.
