The infinitely stacking champs are: 
(remind me if i forgo one)
I think I would personally want thresh since his souls give defense and damage on all of his abilities including his autos.
I'm a kindred main, so....
Plus you can outkite all of the listed champions.
You forgot 
.
Sweet sweet susan
As a bard lover , probably bard.
It would be pretty fun to assassinate someone with sion and titanic hydra.
Probably would be between since I feel they are the most scariest in that situation. While would be pretty good to, I still think Nasus might be able to beat them mainly for the Wither being really really strong but then again I guess that depends on how many stacks are we really estimating for Kindred since they require champion kills (and maybe Baron/Dragon).
Actually, now that I think about it...With the changes to 
would she technically count as another infinite stacking champion as well? Since she gets Armor/MR for every dragon kill which I believe counts Elder dragon as well. I know if she did count, she wouldn't be better then half of the others on the list but just wondering if we should start thinking of her as one now.
stacking would be nasus
veigar
. Maybe because i'm Kindred main, but hear me out:
If I can get 12 stacks in normal game, in 6h game I would be able to get like 4-5 times more right? 40 stacks mean 50% of current hp per aa. So Thresh and Sion are out, Kindred don't care about your hp or armor. Add to this 8% current hp from BORK and 3% of max hp from jg item. Plus Attack speed that Kindred loves would mean he can kill you in 2-3 aa before you can get close. And if you somehow can, he will ult and then kill you.
