As a born-to-be-bred bottom whore I have tried many ways to thank the breeder, and I find there's no one right way of doing it. Especially when it comes to saying it in words, you really have to go by the situation. But there are many ways to show your appreciation without words. First of all cleaning his cock after he breeds you. But be careful, cuz many guys are very sensitive right after. However, you can clean the shaft and the balls without sucking on the head, and that also shows that you're servicing him, and not that you didn't get enough. What I usually do is to put a finger in my cummy hole getting some of his cum out and taste it so he can see how much I like his cum. I also give his cock a gentle grab quietly whispering "wow" or something like that... Give his chest muscles a firm touch with a worshipping look in my eyes... If he's sweaty I'll lick some part of his body, preferably his arm pits, to show him how much I love his man juices... Stuff like that.
And I always wait until he's left and is out of sight, before I get in position for the next one. I don't want to give the impression that he's just one cock out of so many, thus not being special. Cuz every cock and every guy is. Every breeding is unique, and even if you get bred by dozens of guys in one night, it's each one of them that made it a good cumfilled night.
If words are appropriate, I thank him, but I usually say stuff like "damn, you fuck good!" too; again so as not to give the impression that all that counts is his cum. What matters to me is the feeling of a guy using my hole to get off in, and that has to do with more than his cum as the end result.
And, as in all matters of communication, never be fake! If it's clear to both that he didn't fuck so well, was a bit too drunk etc., don't overdo it. Just show appreciation for the fact that he chose your hole to unload in...