IMDN all day every day. It's the complete package, TWD has a few strong points, but is far from perfection. Let's break it down, shall we.
Theme: IMDN - If you like Iron Maiden, then IMDN is amazing on all fronts. If you like The Walking Dead, then TWD somewhat resembles a cheap knockoff of the Walking Dead theme, bad voice acting, content limited to the first couple of seasons, etc.
Art: IMDN - Beautiful art package by Zombie Yeti, one of the best in modern pinball. TWD: Fugly as sin photoshop collage.
Layout/Shots: IMDN - Amazing flow and shot geometry, 4 flippers that you need to use a lot, nice combination of easy and difficult shots. TWD - Poorly executed playfield. Top right 1/3 wasted on meaningless bumper nest with terrible random exits. Momentum killing well walker about as fun to hit as watching paint dry...
Music/Sound effects: IMDN - Soundtrack is rocking! Sound package is fantastic. When you light this game up it feels like the end of the world! TWD - Okay but not great.
Callouts: IMDN - Some of the highest quality voice work in a pin in many years. Custom callouts from two distinct voices (male and female). Humerous as hell too. TWD - Incredible lame and annoying fake southern accent guy...
Rules: Tie. Both of these games have some of the best rules packages in pinball.
Premium Features: IMDN - left ramp lock is rewarding to shoot and adds shot variety to the game vs. the Pro, as does the lifting center ramp. TWD - crossbow is a mixed bag, though the walker-bomb button is appreciated by most. Most people don't even shoot for the bicycle ladie under the left ramp and it's not really fun to do so.
Let's face it, IMDN is an all time classic that wins on every level. If it weren't for Lyman's amazing rules TWD would be forgotten failure, rarely mentioned and often found on the same lists as WWE.