To be honest I'm not expecting an answer to this question. While it sounds simple, I think it's probably quite involved.
For a given channel I want to return one date/time - it will be either the latest channel entry date or the most recent comment date, whichever is the most recent date/time.
Cheers Lee
You can use the Statistics Module (as John said) but the following code will output either the comment or entry data like you requested:
{exp:stats}
{if last_comment_date > last_entry_date}
{!-- Comment is more recent --}
{last_comment_date format="..."}
{if:else}
{!-- Entry is more recent --}
{last_entry_date format="..."}
{/if}
{/exp:stats}