Before the rules change and still now, for me, Call the Skybreaker is the best win-condition/finisher/whatever Mono-Blue has. Rainbow Efreet was #2 and Morphling didn't even make the cut of my deck (it was close recently but I added a Mana Leak...HA!). Now, I see Rainbow Efreet is much worse than Morphling, but I also don't feel Morphling has the bomb level to be the #2 win-condition in my deck. In fact, I changed my decklist altogether and am running 8x creatures (ish) opposed to the 2x I was; I'm currently trying:

4x Sower
2x Kira
1x Wake Thrasher
1x Call the Skybreaker

OR

4x Sower
2x Kira
1x Meloku
1x Call the Skybreaker

Before the rules change, I tested Kira but did not eventually play it in my deck - I found it good but not quite good enough. It's a decent turn-3 play because it is really annoying for the opponent if he or she doesn't counter it. It can be an early clock against TES (I wouldn't play it turn 3 without FoW backup against TES though), a solid poker versus Landstill, or even just come down to block a Nimble Mongoose or trade with a Warchief (maybe). Since I feel this deck has lost a lot of power in its bombs, I think it might be time for me to go back playing Kira.

When I first played the deck, I had Meloku and Morphling as my win-conditions. I found Meloku to be really good against Threshold; the only problem was it couldn't protect itself (I still found most Thresh players sideboarding out the Swords to Plowshares anyway because they only hit 1 creature). With Kira, it can sort of protect itself, but that's not really that necessary because Meloku is really good against aggro and is also a good way to bounce Academy Ruins under Back to Basics (if you run that card).

Also, now with the new rules of no mana burn, a turn 5 Wake Thrasher is pretty good especially if a Kira is already in play (but not necessarily ONLY if Kira is in play). You will be smashing for 6 the next turn, 7 after that, and you will never have to be in a situation with your lands tapped and wanting to do something else because you can just tap your mana end of turn and if your opponent plays something, you can respond; if not, mana poofs away.. In either case - bigger Wake Thrasher.

I was never a fan of Guile - its ability isn't too exciting and it can't protect itself. At least Meloku has a relevant ability and Wake Thrasher can come down to beat a lot earlier with their vulnerability. As for Inkwell Leviathon, I feel it costs too much mana as I'm usually winning with Call the Skybreaker or one of my other win conditions by the time I have 9 mana. It just seems like this would be trying to slow down the deck even more. Just remember, the more time you give your opponent to recover under Back to Basics, the more likely it is we will lose.