Depending on what kind of Spartan, then it would be MJOLNIR Mk. VI. Though it is the most expensive of all the armor a Spartan could use, it is easily the most advanced and is also the same one the John-117 uses in Halo 2 and 3.
It would depend on the spartan yes but it would also depend on what their are doing and where their doing itlike you wouldn't use EVA armor to defuse a dang bomb and MJOLNIR MK. VI is overall the best it is but it doesnt go better for one thing or another its like a jack of all trades and the master of noneall the other armor except MJOLNIR MK. is based for one main purpose