I still think IM gives the broadest base possible for going into critical care. As has been pointed out more than once almost anyone can get up to snuff handling "critical illness" itself by finished a fellowship through any of the pathways. There is much about critical care that is critical care is critical care is critical care - though it is nuanced. For instance, at one of my rotational sites we get quite a few post-op consults from the general surgeons - I can easily run the vent, pressors, abx, deal with fluid balance in these patients, but I don't have the same reflexes on certain post-op complications that someone from a surgical background would have. How should I know when a patient needs to go back to the OR for instance? (I will say if it's so obvious the internist can tell, the patient is probably beyond repair). Based on my experience the IM background guys are simply better at definitive diagnosis and long term treatment plan, especially in the MICU (duh). I also think we are better with sick hearts (not post-op hearts - there's a difference) and sick lungs on top of whatever else is going on. I think SICUs should be run by surgeons and I think it's kind of stupid to stick an internist anywhere near most post-op surgical or train wreck trauma patients - because as stated above, we simply lack those reflexes. The Gas guys seem to be the best at post-op hearts and lungs, including transplants of those organs - though interestingly enough though they are good at the physiology, their lack of understanding on general medical problems itself in any detail, I think puts gaps into their long term management moving out of the perioperative phase. The EM guys in my opinion tend to be kind of like the are in the ED - there is nothing wrong with the critical care per se, but often seem to lack that extra mile in diagnosis and long term planning ("not sick enough for the unit, transferring to the floor, medicine will have to figure out what's wrong - just like the ED, don't have to go home, but they can't stay here").
I'm not hating on anyone here, and, sure, I'm generalizing. We all know that surgeon who could be easily be an internist, or the pulm guy you wouldn't want anywhere near anyone you know or love. Or the gas or ED guy thinking around crazy corners to come up with diagnosis. I guess I'm just trying to point out that we will all come with our own "holes" in training. We will all, once fellowship trained, have the tools necessary to deal with critical illness, and it will be up to us to be both introspective and proactive about these things as professionals and get up to snuff with what we will need to finally bring everything we can to the table wherever we end up working.
I guess, in the end, I've said a whole lot to simply point out that if you're asking who makes the "better" critical care doc from IM, surg, gas, or EM, I think you're asking the wrong question and looking at it a bit backwards. We all need to check a little more ego, learn a little more from our other specialty homies, and deliver the best patient care we can.