The Rinnegan is not a KKG, so it's not a viable option.
As for the poll, the two best are EMS and Mokuton, but Mokuton's sheer versatility, properties and scale of techs make it the best.
Rinnegan is one of the three dojutsu, which includes EMS (sharingan), so its one of the bloodline limit techs.
The Rinnegan is reputed as the most exalted eyes amongst the Three Great Dōjutsu (三大瞳術, San Daidōjutsu; Literally meaning "Three Great Eye Techniques").[2] It is characterised by its ripple-like pattern which spreads over the eyeballs, with light purple irides and sclerae. (narutopedia)
I'll go with the EMS because blaze release comes with it and with some senju dna(hashi dna) the rennigan and wood release is mine
You'll have to start with the basic Sharingan first before going all the way to EMS.
It's not that easy to get Senju DNA. Even if you did... nothing would happen until you turned really old.
THEN you'd unlock Rinnegan.
I would make my own Kekkei Genkai called, "Swift Release"
They'd have powers like Flash, and have various Jutsu that'll increase your speed.
They would also have Flash Step like you see the soul reapers do.![]()