Which armies/detachments have the highest effective model diversity at the top competitive levels?

I often hear discussions about certain factions being highly effective at competitive levels but are restricted to a small domain of possible list configurations. This effect also appears to lead more casual players to view these armies as weaker in casual play, as not adhering to a strict subset of model choices generally causes much lower performance. In this post, I do not intend to discuss these armies but instead look at the opposite case.

I'm wondering about factions that have the most freedom in list design and gameplay strategy when played at the competitive level. The things I'm wondering your opinions on are as follows:

• Which armies (or detachments), at the top competitive levels tend to show the most diversity in list building or model selection while continuing to remain competitive?

• Which armies can effectively utilize the largest number of different game strategies without suffering performance at competitive levels?

• Which factions offer the player the most choice in how to play the game?

Feel free to interpret these questions broadly, I'm interested in the general community's opinions on the topic.

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Additional follow-up questions to consider:

• At the top competitive level, does list diversity really exist or is there truly an optimal list for every faction?

• Conversely, is the idea that certain factions have low competitive diversity really true, or is it just a meme? E.g. Can a good enough player win with any list configuration (that is minimally sensible) for any army if they are practiced enough with it?

• Which armies can be effective at the competitive level without utilizing "new" models (i.e. models that came out since 9th edition) or without requiring a large reinvestment in models due to rule/index/codex changes?