Difficult to say.
If it comes to shear power I would love Ace's Mera Mera no Mi because I just love fire and Enel's Goro Goro no Mi is just awesome, being lightning itself.
In the Zoan class I would like Marco's Phoenix Fruit and Rob Luci's Neko Neko no Mi: Leopard.
In the Pharamecia class I would like Daz Bones' Supa Supa no Mi fruit changing your entire body into a sword xd Because Luffy's Gomu Gomu no Mi gets so much attention it would seem like a very good choice, though I doubt anyone of us would ever found so many uses for it as Luffy.
But if it comes to the most useful of abilities, believe it or not, I would say it's Wapol's Baku Baku no Mi. I mean two of the biggest problems of every human life are practically solved. One food. Everyone needs food, if you can eat whatever you want you can even survive in the freaking desert. On top of that you can even use the abilities/qualities of the things you just ate. So maybe you would be even immune to things like poison if you would eat it. Two money. Because you aren't forced to buy much food and you barely have any wasted food you save some money + you don't need to buy expensive stuff. I mean if you find a broken TV, by eating it you can repare it somehow and voilà you have your own TV. Then, just like Wapol in the manga, you can start selling it.
Seriously that's one usefull ability.