everybody has their own idea about what the best attacking nuke is, but really the best nuke is different for every attack you do.
what i do is just build whatever attacking units i have resources for (horses and slingers balance out resources fairly well, with the odd hoplites or chariots thrown in when i feel like it). i always say that the best nuke is the one which is built, as opposed to being 'under construction' or only half built because you're waiting for the right resources.
if you want to get theoretical and technical then pure nukes are always better if you have the correct one (pure nuke = all slingers, all horses or all hops/chariots). if you find the weakness in the defence then you send the appropriate nuke and you will always have lower casualties than any mixed nuke, e.g. if the defence is short on distance defence then you send a pure slinger nuke. the only exception is against a perfectly balanced defence, in which case any pure nuke will have exactly the same casualty rate as any mixed nuke, but other than this rare case, pure nukes will always be more effective.
that being said, pure nukes are more of a chore to build because they tend to drain one resource and not another, leaving you unbalanced and needing trades to get your resources back on track. also , building pure nukes means that you need at least 3 land attack cities in any given area so that you can choose the correct nuke after spying. all in all it's far easier and quicker to just build whatever you can
EDIT: one exception i like to do is pure sharp nukes (hops or chariots) because then that city can double as land defence when needed.
