Right up at the top of the list, from a vintage pro-am title, "Southern Redhead Gangbang" - the scene where this fantastic little cuckoldress is airtight, near the first 1/3 of the video is one of the hottest things i've ever seen.
In that same realm, a softcore short, "Beverly Hills Bordello: Things Your Wife Won't Do" - I can't recall the actress' name, but it is a cuckold-theme short, where the wife is seduced, willingly, while her husband watches.