We don't know yet. We will figure out what is the best during the beta in order to point out what is better to god roll now.
As an exemple, if you can't have
this kind of roll in the beta, then roll as much as you can in live in order to keep the advantage forever. (That's why grandfathering is a mistake)
edit : Don't forget to pray RNGsus and make sacrifices for him at engineer settlement.
I doubt if these rolls will be obtainable anymore, with having either superpen or casca, if we talk about rails. Just look at latest vids that shows "bit" of new system.
I strongly suggest that anyone should try obtain these rails soon as possible, as secondary rolls (low heat or so) will gonna be replaced with bunch of new "experimental" effects for all weps, so having low heat raigun with superpen and casca wont be possible anymore as after change, if SOMEONE(Read: all PVP'ers) would like having lower heat on thier rails, then there will be new experimental effect for it instead.
For weps, apart of railguns wich are most obvious, there also these focused/long range PA, with lower heat as well, since its lower heat is what makes them so viable now. After change, there wont be possible to create meta weapons with same stats as before.
There is list for those who would like know what is best to roll right now, before it will be not anymore possible:
1: Long range rails with superpen or casca or even slug, all with low heat (12-14 heat instead of 20). Any of them with 5km + is plently good enough.
2: Long range or focused PA with low heat, same like with rails. TLB and dispersal. If there is more than one PA in loadout, matching thier speed should be done.
3: Efficent weapons with 20%+ dmg boost. Especially PA's with TLB and dispersal. There is no weapon wich would not benefit from it.
4: Overcharged weapons, with lower heat. Again, with good experimentals on them. MC's with corrosiv or emmisiv, autoloader. PA with TLB and dispersial etc.
5: Emmisive/scramble long range lasers with low heat.
Mainly is about having lower heat from secondary, and with must-have experimetnal along on certain weaps. Increased dmg on efficents, to having that sweet 20% increase, again with experimetals along.
Apart of weapons, biwave thermal resistant shields might be thing to do as well, since they can get nice 20% regen bonuses, along with good resists. Also Pplant mods of G1 with no downsides. Especially those godrolled g1 ones. Either LE or OC or armoured. Unless if internals would had possible to get "experimentals" on them as well after change, wich is not certain till now.
Do it before its too late.