Early levels Fire/Rad and late levels Fire/Kin.
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
This is the blaster forum, there is no rad or kin.
A controversial one to be sure! 
To be honest tho, this question suits me as I tend to play 99% of the time solo on all my alts with only very rare teaming.
I have not played anywhere near all the possible combos and I tend to favour single style blasters (Same Primary & Secondary) For what its worth (who said not a lot???) I will give you an evaluation of those I have played.
Fire/Fire is a great damage combo but it is also therefore an aggro magnet of enormous proportions! I like it lots, it "feels" powerful but apart from Ring of Fire it has no real damage mitigation. You either have to play very tactically (slowly) or blast like Hades and hope the bad guys fall before you do. If debt is an issue for you that last tactic will probably not suit you!
Energy/Energy. My most recent combo this. It fels pretty safe to be honest, as with all the knockback not much gets near you, What does, does not stay there long and even mobs that fight from range spend most of their time getting back to their feet! Damage is nice as well although its not quite in Fire/Fire range. To be honest tho I almost find this TOO safe. 
Elec/Elec. This one surprised me a lot. Not taken it to high levels and I went for it mainly coz of the end drain factor. Some decent holds with Electric Fence and Tesla Cage but, what really surprised me was its damage! I don't know what the actual numbers are and I MAY well be wrong, but it feels like its possibly the best damage dealer of all. Fires DOT MAY have the edge but single target wise Elec seems to be BRUTAL!
Ice /Ice A great mix of ST attacks and ST hold/immobs, making it easily as safe as Energy/Energy. What I like about it tho compared to Energy/Energy is that things still get "up close and personal". With all the debuffage tho its usually survivable
So, which is best?
For me its "Ice/Ice Baby". Decent enough damage, lots of good control and with the debufs/slows you can play from range or get a bit of blapping in, whatever suits your style or your mood. 
/em "stands there in his nomex suit and aits for the flame throwing to start."
[ QUOTE ]
This is the blaster forum, there is no rad or kin.
[/ QUOTE ]
Well if I cant go with not a blaster and have to pick something less logical I'd say either
Elec/Fire hazard hunting
or
Ice/Elec missioning
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Ice as primary for sure, the holds to stop you being mezzed/hurt badly are nice. For secondary either energy or electric.
Most blasters solo with ease these days. I'd go ice/nrg, fire/nrg, or what the hell, even fire/fire
Loving my Fire/Energy. It's awesome at everything ive put my hand to. PvP, Solo, PvE.
Hero Plan by Mids' Hero Designer 1.30
http://www.honourableunited.org.uk/mhd.php
Holly Steel: Level 48 Science Blaster
Primary Power Set: Fire Blast
Secondary Power Set: Energy Manipulation
Power Pool: Fitness
Power Pool: Speed
Power Pool: Leaping
Power Pool: Medicine
Ancillary Pool: Cold Mastery
Hero Profile:
Level 1: Fire Blast -- (A)
Level 1: Power Thrust -- (A)
Level 2: Fire Ball -- (A)
Level 4: Build Up -- (A)
Level 6: Hurdle -- (A)
Level 8: Hasten -- (A)
Level 10: Bone Smasher -- (A)
Level 12: Aim -- (A)
Level 14: Super Speed -- (A)
Level 16: Health -- (A)
Level 18: Blaze -- (A)
Level 20: Stamina -- (A)
Level 22: Combat Jumping -- (A)
Level 24: Super Jump -- (A)
Level 26: Stimulant -- (A)
Level 28: Aid Self -- (A)
Level 30: Power Boost -- (A)
Level 32: Inferno -- (A)
Level 35: Boost Range -- (A)
Level 38: Total Focus -- (A)
Level 41: Snow Storm -- (A)
Level 44: Frozen Armor -- (A)
Level 47: Hibernate -- (A)
Level 49: Conserve Power -- (A)
------------
Level 1: Brawl -- (A)
Level 1: Sprint -- (A)
Level 2: Rest -- (A)
Level 1: Defiance
Code:[/color]| Copy & Paste this data chunk into Mids' Hero Designer to view the build |
|-----------------------------------------------------------------------------|
|MHDz;1512;407;544;|
>-KE5,MJPS`0O!O\#_D!%TLKO^BMCY!"<PKT;I(E%1@YV.XA(/SME6RYWF9)?Z#D,4@S.QJM+.^P:U_
JH?[`KM>M>10/*HYV;=-<-X<!L;&'HT9S1/O4U/V`71QM=8>;:61?#7;GZV9?&WWY:NK!&=@W\XD=FL
%N]6"P[^WA@GBR[UA?M#G;/9[T41NTSVUS<I7>\QI'J9T_POW&,1T3,2H'8II,!&'O\_Y?WO)BY <$-&
`\S[Q79I+CAL[6^L(KSQ<I7=K;[Q5<DW[S(;4"24(!;@F^!9!0^)%>0E,(GRIFB(!Z5\V!!!4DJ_;[9
5B5)*L$I6`X)1)&Y%-R#))6%4W`/TE'I6\KV(DE2\%"R0Z6G[@^M8`I%%(53<`^20Z63PXU"I6M/%3@
%VXLB_5"Y4["NJYPH2F>EF*)<%//S??^&S+S\@_?//]RY(>+G_B1BK4C(][]-^FY`-8_\(8A23F^!&?
.`64`14`64`2%@&7!Q5F&QZ?Z+$H(%A%((I1`L(2P%8>E)OQ26 R\3B4`4L`L;1-W-6[S$`
|-----------------------------------------------------------------------------|
Quickly done my build, i'm only level 42 at the moment, but i love this build.
Rather than de-rail the thread though. Fire/Energy from my experience is very fun, Solo, Group PvE, and PvP. But everyone has their own tastes! 
There is no 100% right answer on it but in terms of defeat one next one with control I fancy Ice/Nrg. In terms of AoEness and control maybe Fire/Ice and be an active defense mini tank. I did level a Fire/Fire, Energy/Energy, Ice/Ice together for a bit and consecutively soloed Frostfire EB at lvl 7 with them (I like to see balance). Fire dropped Frostfire between the time his pets despawned and respawned, energy slightly better but ice needed some control, being slowed by frostfire and having long rechg attacks oneself didn't help. Either way though they all paid of nicely they just need to be played slightly differently to get to the same end. Ofc instead of going, say, ice/nrg one might want to see what the psi looks like in i12.
He will honor his words; he will definitely carry out his actions. What he promises he will fulfill. He does not care about his bodily self, putting his life and death aside to come forward for another's troubled besiegement. He does not boast about his ability, or shamelessly extol his own virtues. - Sima Qian.
So here is the question nobody likes and tends to have a go at ppl that ask it, what is the best/fastest combo for solo pve, with very occasional grouping,
flame away.