I can highly recommend the new hybrid NiMHs as the best investment you'll ever make for your flash unit(s).
The reason is that they hold their charge for a long time. NiMH rechargeables are going to give you faster recycle times than anything else on your list, I believe. And they may actually give you more shots on a single charge than anything too with the possible exception of the Lithium AAs which are great, but way too expensive because you only get to use them once.
I'd save those for emergency use. Put a pack of them in the camera bag to use if everything else is dead and you don't mind blowing that kind of money so you can shoot. They have a great shelf-life.
But, the new hybrid NiMHs, such as the Sanyo Eneloops, have a tremendous advantage over the standard NiMHs. And that is that they don't self-discharge anywhere near as fast. The claim is that after six months of sitting, they'll still have 80% of their charge! Normal NiMHs, for me, are totally dead in a week or so whether I use them or not.
So you must charge them up immediately before you use them. And that can be a pain. I sometimes go quite a while between uses of any given flash. And I just hate it when the batteries are dead - and they almost always were for me with normal NiMHs.
Alkaline AAs are not good. They have a fairly high internal resistance so they heat up if you shoot a lot in a short period of time and they make your flash take a lot longer to recycle between shots.
The Lithium AAs are nice, but they're outrageously expensive. As I said, use them for emergency backup.
As far as I know, the Titanium AAs are not significantly better than the standard alkalines.
And the normal NiMHs are dandy, but you've got to recharge them right before you use them to have a good charge.
So for me, the hybrid NiMHs are the real choice these days. They are truly amazing in my opinion.
Here's a thread about the hybrids:
http://forums.dpreview.com/forums/read.asp?forum=1018&message=22975952
My experience is only with the Sanyo Eneloops. And I can't say enough good about them. They are the ideal battery for an external flash in my opinion. The other brands may be equally good, but I can't vouch for them because I have not tried them myself.
Even if the stated amp-hour rating is lower, believe me, you'll get a lot more use out of them because you'll actually get to use the capacity they have instead of having it drain away before you get to use it
--
Jim H.