The cheapest OLU (Offensive Land Unit) nuke and the fastest to build is the slinger one and the most common.
But the best are Mythical Units nukes, either harpies, manticores, erinies or griffins. Harpy nukes are the fastest to build and are common in early game, but later on the best would be manticores, erinies or griffins nukes, depending on what defense the city you're attacking has. Flying myth units (manticore, harpy, griffin) are very useful on a conquest world to break sieges, but also have their uses on revolt worlds to land a revolt without clearing the navy.
As for defense, as I said in another thread, there used to be a ratio on building DLU (Defensive Land Units) nukes, but it has changed a while back and no one has the will/math abilities to make it out again that I know of
I wouldn't recommend myth nukes for defense as they're basically useless if the other city doesn't worship the same god.
All this depends on your play style, some disagree with what I just said, there is not one true way of playing, only different points of view