This is a fantastic thread - great information.
I've thrown a handful of Storm balls in my day, many with similar layouts. So far, the stronger cover I've ever had from storm is the one on the original IQ Solid (non-tour) - the PFT Solid (as per Storm website). After that, I think it's R2X, NRG, ERG, R2S, then Reactor.
R2X - Original VG, Gravity Shift, and Marvel Pearl have all been early-ish reading and very smooth but continuous off the spot
NRG - VG Nano, VG Nano Pearl, and IQ Tour Nano both get down the lane a touch longer than my original R2X gear did. Note: I had identical drillings on both versions of the VG and the original was just a touch earlier rolling. My IQ Tour Nano is like butter when there's enough head oil. I like this coverstock a LOT - a lot lot.
ERG - I have a Zero Gravity that's a lot weaker than I thought it would be. I got it drilled identical to a Gravity Shift I still use, and it's just really wonky reacting. It works well on less oil than I thought, and has trouble cornering in carrydown. I use it intermittently at best. My dad owns a Byte, and there's almost no hook phase...the ball seems to go into a roll almost immediately after reading the dry. And from what I've seen out of the Crux line, it's more of the same. This cover seems to be for guys with lots of hand, or lots of end-over-end roll.
R2S - I throw 2 IQ Tours, IQ Tour Fusion, and I used to throw a 2Fast and an old school T-Road Pearl. All of these balls have worked wonderfully for me. For me, this cover has more length than the R2X/NRG/ERG, and it also has the most motion downlane. The R2S solid on the IQT is EASILY my favorite cover from Storm. A Wipe Out is easily in my future, lol.
Reactor - I never threw this cover. I saw a Tropical Breeze work for a guy with about 12000 RPMS (ok, maybe 450), and realized I don't have enough hand for a ball with Reactor on it.