2000ad - either dead tree version or the iPad app os pretty good (if you've got an iPad, obviously). Cheaper than dead tree and back issues are easily acquired so you can quickly get up to speed. OR, Rebellion (publishers of 2000ad) have a massive collection of trade paperbacks available collecting all the Judge Dredd, Rogue Trooper etc etc stories together.
Walking Dead do a good number of trade paperback collections but I went the compendium route - pretty much 50 issues in each of the two compendiums currently available.
DC did a restart a couple of years ago so Batman and Detective Comics started at issue 1 again. Trade collections of the first few arcs are already available but the actual comics are easy to find as many places overstocked.
OR, if you have Android or iOS at all, hit up ComiXology. Brilliant digital service, regularly has sales but the comics on there tend to be cheaper than dead tree versions anyway. Lots of collections as well as free issues to get you going and they don't take up the space that 'X' years of collecting brings (15 comic boxes and counting, here...)