Essence of Undeath to Water, by far worth paying more for! I can make 8g per transmute, can do one per day, that is 56g per week, figure even if you pay 200g, that is one month for a few seconds a day. After that it is all profit. Now your server economy may be different, it is hard to say. I bought it for 60g and it paid for itself many many times now. I don't sell much now because I am making water to make Hide of the Wild for guildies, but that is the one and only transmute that I would currently pay for.
I am constantly laughing at the prices for transmute Water to Undeath. Whenever someone asks me if I can do water to undeath, I tell them, sure, send me your essences of water and I will send you as many Undeath as you need, right now. I see that transmute going for 200-300g. OK, so it's rare... so what. I take a 10g semi-rare useful item and turn it into a 1-2g item. Hmm I'm not the best in math, but I can see that doesn't make any sense whatsoever.
If you watch the AH, you will probably find Essence of Undeath selling in stacks of 10 for lower prices than singles. Bid on any you see that have low bids, if you lose, no problem, it takes you ten days to do one stack, soon you'll have more than you can do any time soon.
Transmute Water to Air as someone said is the next best money maker, though you need to start with a lot more. When I started doing it, some wise-guy decided that raising the prices on Essence of Water was a good idea, so he bought them all up, and put them back on for the same price as essence of Air. I had been buying up the good priced ones to transmute. When no one bought them, the prices went back down.
I don't even consider doing Arcanite transmutes anymore. It is the same timer as the essence transmute, but takes two days to cool down. Consider I can make 16g in two days and about 4g for one Arcanite transmute, four times the profit for Undeath to Water. Also remember, as someone else said, 10g for water to 20g for Transmuted Air gives you 100%profit. 2g for Undeath to 10g for Transmuted Water gives you 500% profit. Once a day.