That's pretty hard to answer, but I will try.

1st: Master a FEW powers, by putting a LOT of enhancements in them, instead of dividing everything everywhere.  Having an attack with 1 or 2 acc, 3 damage and 1 rch or endred, it is better than having 3 attacks with 3 damages each and nothing else.

2nd: Build up and Aim are very nice, and they are never obsolete as you level up.  (maybe not both at low level, so I would try to start with build up myself)

3rd: Think of a way to work professively toward stamina, not at every cost, but eventually, by 24 or 26 if possible, because after that it might get quite far.

4th: I like super jump myself, I know peoples in PvP focus usually with superspeed and super jump, me personnally I prefer flying in PvP, as you can get out of reach permanently and choose who to fight and where...yet I am not really good in PvP...but as a blaster, you have to think about killing an opponent in 2 or 3 blows, falling on them from above seem ideal to me as most peoples won't be able to get to you ever.

5th: Try to take some powers from both aspects, to diversify your damage sources.  See, as a fire/device, you may end up with mostly fire-based damage, try to get a non-fire source, maybe not at 17, but eventually.  (I don't know the devices at all, but some must do damage)

6th: I wouldn't slot the fire rain, if you take it, the main use I see to it is that it disperse enemies in PvE, while they run they don't attack you...and they scatter which give you some PvE options.

7th: As a blaster, you died fast.  So both in PvE and in PvP, select the right timing for attacking, not been attacked.  Again, fly is ideal IMO as most peoples won't get high enough...not to mention many scrappers/brutes/tankers have no range attack...so by moving a little, you can avoid all their potential attacks, I can't explain to you how to fly to avoid a superjump as I don't understand it, but I saw it, and it is impossible for someone to auto-follow and hope to fall on someone who fly at the right moment...if you know how to move.  YOU HAVE TO ATTACK, not been attacked.  You can increase the range of your attack to better enhance this advantage of the blaster.

8th: You should get stealth powers in PvP, and maybe stealth detection powers as well...maybe not as early as 17th however.

Best PvP built...that is hard to say, and it depend...do you mean 1 vs 1 or 8 vs 8?  See, a scrapper is very good on it own, but in a group you prolly don't want any scrapper.  So again, it depend what your goals are.  Been able to 2-shots some folks is definitely nice, but it is more complex than that.  For PvP I would say that generally, focus on heavy single-target damage, and for PvE on AoE damage...but again, it is also nice to kill bosses fast in PvE.

You also should consider all the temporary powers in PvP, they make a HUGE difference.

Fly is absolutely horrible for pvp on its own for most people. There's simply too much -fly attacks out there (being a /dev blaster u know this since you have one yourself). Almost all my villains have a -fly from my spines stalker to my corrupters to my brute (webnades). Now Teleport + Fly is fairly decent combo which is what most /dev blasters go with. That being said Fly does have some uses like the above poster says it can be fun to use when there's no one with a -fly attack around (but keep in mind u can even purchase a -fly at sirens however not sure how many players do this). anyway Teleport is a great, natural complement to fly. If you get hit with -fly then you can simply Teleport away until the -fly wears off

TP Foe is something good to pickup for  a /dev blaster. setup your mines somewhere and then simply TP Foe your victim unto your mines. easy kills. Add on tactics for +perception bonus this way you can see stalkers. You already have a base +perception bonus in your secondary thus adding on tactics is all u need to become a stalker hunter. once revealed you'll find stalkers fairly easy to kill. Stalkers are a big problem in sirens especially the EM/Regen FOTM stalkers so /dev blasters are a welcome hero for stalker culling. If you go this route make sure you spec all this in a siren's call build (lvl 30 build).

Post 40 Blasters are anything but easy to kill thanks to the high +Def bubbles epics. I've had some long fights with some good ones on my EM brute. Now to think they had the high +def bubble epic + aid self (medicine pool)

The best ATs for pvp is the basic MMO rule for PvP. Range. Ice/EM blasters and Rad/psi defenders are the FoTMs. Ice/Rad corrupter is good for a villain but no where near the god that is rad/psi. problem is rad defenders get -40% resist bonus no villain gets such an outrageous advantage. Stalkers are good in sirens but not as good in RV no thanks to Focused Accuracy. So stalkers go EM/Regen because Focused accuracy cuts through high +def builds like ninjitsu, sr, and nrgy. but this is not law of course I still maanged decent in RV on my stalker. Ice/Cold/Mace corrupters are pretty fierce in arena you will have enemies totally immobile. ice/rad corrupters decent in sirens call but sorta lose their potentancy in RV cause most heroes have resistance shields in their epics. problem is corrupters dont have unresistable debuffs like heroes do. Em/ELA brutes are very good and I hear EM/Fire is also FoTM for brutes. problem is brutes lack range capabilities thus they can be kited easily also. I do very well on my em/ela brute I can kill scores of heroes but like any melee I am easy prey to rad/psi defenders even in my tier 9 defense. of course there are no absolutes in pvp especially in a complex one like CoX. So yes I have killed rad/psi defenders in the zones but its an unusually hard fight. another bad thing against a brute if they get kite they lose fury. I lost an arena match to a dm/regen scrapper that did this to me. I'd still rate EM/ELA brutes a really good pvp toon especially for zone pvp (burst potential is insane when fury is built). for team arena/pvp its one of the best 'taunt bots' because it has no defense holes

So best toon hands down is rad/psi defender. they can self heal, they can debuff the best, have nice loose builds (you'll have both superspeed + superjump in a sirens build!), get best tactics numbers, best pvp teamrole (call targets, place debuffs on them just gotta keep target in range thus u need maxed out superspeed,hurdle+CJ+SJ to keep victims close).

There are other sleeper builds though like Grav/Rad troller are just baiscally any sort of /rad troller. mind/kin troller can be nasty but lacks survivalibility (no good reliable self heal).

BS/Regen scrapper is good but can easily be pinned down by ice/cold/mace corrupter or ice/kin/mace corrupoter. Ice/kin corrupter is the best teaming build in sirens but lacks survivability (no good reliable self heal, transfusion requires u to come in too close and doesnt work good on kiters).

Ice/EM Tankers used to be FoTM but now they easy to beat by EM/ELA brutes. but keep in mind in team pvp all meleers do is Taunt mostly unless you're a Spines scrapper (which are good for pvp in various ways too). Empath defenders are also good in team pvp but no good 1v1 obviously. however in sirens call any sort of Empath is a welcome AT I'm sure. Clear mind alone gives hold resistance and +perception

 FYI one of the best pvp guilds is Freaks Of Legend on freedom I believe (they might have a satellite guild on liberty).