Dean USA RC7 by far for me. Most comfortable and best playing guitar for metal/shred ever made, IMO.
Runners up to the RC7 would be various Ibanez and EBMM JP7. Both the Ibanez and the JP7s have fairly thin necks and play pretty comfortably... I'd definitely take them over Schecter/ESP/etc., but the Dean RC7 has an even thinner neck, deeper cutaway, larger frets, and a very flat fretboard. It's basically everything Ibanez fans like about Ibanez guitars taken to further extremes. The playability is just untouched. I'm excited to try out the new 8 strings that will be debuting at NAMM... should be the most playable 8 strings around...
BTW, I've never understood why people think certain necks are too thin. They say they get hand cramps from thin necks, but I see that as a result of poor technique. Keep your thumb on the back of the neck classical style and keep your hand RELAXED, and you shouldn't have any problems with cramps.
The Ibanez Wizard I necks already have a 430mm fretboard radius (approx 16.92 inches) and the trems are pre-radiused to that too.
I can't imagine the RC7 actually has a flatter fretboard, it's probably about the same, since generally you're not going to find a trem with a radius bigger than 430mm at the saddles.
I know Cooley was playing Ibanez prior to Dean, and I don't think the guitar he had made for him at the LACS is all that much different to the Dean.
Yes, slightly bigger cutaway, and obviously the RG body, but otherwise it's fairly similar (same fret size, same pickups, both have lo profile trems etc etc).
I've yet to find a 7 string neck that was too thin fortunately, but for some 6 strings, yes, some are just too thin.
I don't have issues with hand cramping, it's more so the way it feels. It's only the really really thick necks that cause hand fatigue for me.
Really thin 6 string necks like the Ibanez Wizard I necks (and the even thinner Super Wizard necks) just feel like there isn't enough wood there to hold onto.
Whereas, I'll pick up RG1527 or 7620, or 7421 (basically anything with the Wizard I 7 string neck) and it just feels right and feels good.
The extra thickness doesn't slow me down at all, and the fact it just feels better is the icing on the cake.