It depends. If you want CFW, the best choice would be a CECH-25xx with MinFW version 3.40 (which are the ones manufactured before December 2010) or 3.56 (which are the ones manufactured before February 2011). Those consoles are really hard to identify by only looking at their model number. It often requieres you to look the label underneath (no guarantees) or run the MinVerChk.PUP.
I you want HEN, then the CECH-30xx are the best as they come with the 65nm which translates to better temps, better power consumption, less weight and more easily serviceable (if your BD drive goes bad, you can simply swap it for another one of the same model and it will work out without the need to remarry it).
I wouldn't recommend the CECH-4xxx as I've always found them to be very cheap in quality and design, despite having the best energy consumption-temperature ratio of all models. But that's my personal opinion and taste.