I'd like lightning nature because of the 3rd and 4th Raikage lightning release armor tech , and if I had to pick a nature kekkei genkai, I would pick magnet release. If I had to pick a nature kekkei tota, I would definitely choose dust release.
It's a tight race for me between water and lightning
Water because of how versatile it is. As seen when Kakashi & Itachi first clashed, one used it for lightning fast offense and the other for just as quick defense. It can be used for more purposes than most.
Then there's lightning. Not as versatile but I think a the Raiton Rasengan would be the greatest thing. I would imagine it would look like and be far more devasting than Street Fighter's Ryu's Hadouken
Lightening. There is not a technique more cooler than Kirin. Chidori and its variants are amazing.
Katon. Uchiha Katon attacks are quite fascinating.
KG - Amaterasu. Hands down the best KG in the manga. The metaphorical connotations alone make it worth ones while. It's right up there with Kirin for me.
Honourable Mention: Lava Release from Mei, which is a combination of Katon and Doton.