What new gen Challenger is best and Why?

I own a 2006 Charger SRT-8 and a 2012 300C Hemi and have been a big part of the Modern Mopar community for the past 11 years.

All of the modern Challengers are good but you have some choices to consider (non-SRT since that's what you said you were interested in)...

2008-2010
3.5 L V6 - 250 hp
5.7L V8 - 370 hp with MDS (runs 4 cyl on cruise, 8 cyl on power)
NAG1 5 spd automatic tranny (nearly bullet proof HP, but a bit harsh)
Tremec TR 6060 6 spd manual

These cars are good, but the weak spots tend to be in the front end (tie rods, strut tensioner bushings and ball joints). These cars also have relatively antiquated electronics in the NAV, sound systems and UConnect Bluetooth that are prone to failures. These cars also have "open" PCM so that tuning is easy using the DiabloSport tuners.

2011-2014
3.6 L V6 (Pentastar) - 305 hp
5.7L V8 - 370 hp with MDS (runs 4 cyl on cruise, 8 cyl on power)
NAG1 5 spd automatic tranny (nearly bullet proof HP, but a bit harsh)
Tremec TR 6060 6 spd manual

These cars got some improvements to the suspension and brake feel. Steering was also switched from power hydraulic to electro-hydraulic. Electronics were upgraded also with large touch screen options. PCM received progressive locking by Chrysler from 2011-2014 but all are tunable now.

2014-2017
The engines are the same as 2011-2014 but now all of the automatics are the 8 spd Torqueflight 8. The 6 speed manual Tremec is still available. The automatics have the new improved electronic shifter. The larger 392 Hemi engine is also available on non SRT models

The front suspension was completely redesigned and has eliminated most of the issues of durability seen on earlier models. Brakes are upgraded too. The Electronics are now at the top end of the industry standard offering. There were also body changes that included fascia, hood and tail lights. These cars are not tunable without PCM replacement.