Dear Miss Snark,
Last year I had a few literary agents request the first manuscript I ever wrote in full. All of them ultimately passed for varying reasons, but none of them actually said "your writing sucks". I have since written manuscript number two, which I believe is much, much better and has a wider commercial appeal than my first manuscript.
I would like to approach these agents again as they were all in my top ten list. When I send them a query letter, should I mention the name of the first manuscript they requested in full to refresh their memory? Is this worth doing or am I better just to pretend they'd never read my first manuscript?
No, no and yes.
Absent any sort of comment that is "send me what else you have", most agents probably won't remember your name unless reminded. Why remind them that they passed before? This is a new novel, it's better, being passed on previously doesn't give you a leg up. There's no bonus for telling them, and a lot of down side ie "oh yea, I do remember that novel it sucked even though I didn't say so".
I'd much rather read a query and pages with a fresh eye than knowing I'd passed on previous work, much like candidates for Miss America don't tell you how many times they try out before they win (I was amazed to discover the answer can be ...lots).