So yeah, I'm still turning over the cliffhanger in my head, from a narrative perspective.
I'm still in favor--say, 40% certain--that Morgan is the most likely choice for bombing victim, for narrative reasons and because Morgan could use a plot. Morgan, also, as former bomb squad guy, would be an ironical choice. A counterargument is that the pullback from the driver's-side mirror of Morgan's vehicle is totally intended to make us connect the exploding vehicle with his vehicle, because a cut from one camera shot to a more distant shot from the same angle creates visual continuity. And we know how CM loves to use tricks like that to transition scenes....
Emily could also use a plot, and if you ignore the fact that it's obviously not her vehicle, she's a likely candidate. Garcia, Reid, Hotch, Rossi, and JJ don't really need another plot thread, and I don't think the fans are attached enough to Rossi yet that putting him in danger will have the desired effect. Garcia and JJ are on the passenger sides of their respective vehicles, and as such more likely to survive blasts aimed at the drivers. Would they hospitalize Garcia twice in one season? Tune in and find out!
And how many times do you have to drink for seven interchangeable Collapsible SUVs?
Hotch, also possible--we don't see him get into a truck, and the truck that blows up is paused at a red light, not parked at the curb--but Hotch was last year's cliffhanger, and he's already got enough problems. However, we're really overdue for Hotch backstory, and the CM way is to rake a character over the coals and tell us about their past, all in simultaneity. Also, we're overdue for Emily backstory, and likewise. (JJ three, but blowing up the pregnant lady seems like Easy Angst.) Both of these are an argument against Morgan, of course, because we know a bunch about his history.
The boom-truck does not match Emily's or Reid's vehicles, so if continuity is preserved, it's probably not either of them.
There's also the possibility that the truck that was detonated was a ringer, intended to fool the Domestic Terrorist Conspiracy into thinking their booby trap succeeded. Because as coffeeem pointed out, the last thing we see Reid doing before the explosion is ducking under the dashboard of his vehicle and then sitting back, slack jawed, with that expression he gets when he needs to increase the airflow cooling his brain for heavy crisis-mode thinking. And Reid, of course, is visual-thinking LLI-guy... well, he's the single person most likely to notice a little stripped wire beside the ignition of his vehicle as he sticks the key in, isn't he?
How fast can you dial that cell phone, Spencer? You see things other people don't. Did you see enough, soon enough, this time?
...which leads us, of course, as a total aside, to why Spencer Reid participated in spelling bees as a kid. Spelling bees pay, cash money and scholarships.
You can't play poker until you've turned 18.