Lightning fruits awesome but do you short out is u touch water.
In terms of sheer awesomeness, Ace's Mera Mera fruit wins by far, for obvious reasons, but when it comes to potential awesomeness the Bara Bara fruit beats even the Gomu Gomu fruit.
I mean, look what Luffy's managed with the ability to stretch a bit - were the Bara Bara fruit combined with the Rokushiki that Cp9 use, it's user would be damn near invincible, and able to fire off fingers like bullets, create a shuriken simply by holding one knife and spinning it..... the possibilities aren't quite endless, but they'd easily keep you amused for 464 chapters of manga.
To me Logia fruits are the coolest ones. Like Smoker's Moku Moku or BB's Yami Yami. Though seeing Luffy's ability to change the way his body works using his Gomu Gomu is awesome. Seeing Luffy's gears opens up more possibilities for Luffy to alter the inner mechanisms of his body using the Gomu Gomu.