Wunderlist is very nice for GTD
I use Wunderlist for GTD b/c it is simple and easy to use.
OmniFocus is good too but expensive and without freemium model.
Yes, the iPad is great for OmniFocus -- and for reading PDF files (iBooks)
I echo Barb's enthusiasm for the OmniFocus app and the iPad in general. iBooks makes a very nice reader, and I also use it to read PDF files that I "print" from various sources on my computer. I would probably use the iPad more, except my laptop is a very lightweight MacAir, and I usually don't want or need the extra weight of the (older version) iPad. Despite that, I never regretted the investment.