My favorite piece to mate with the Cloud, has to be any piece I've used so far. I was pondering this answer as I read through the other recent posts and I just cannot decide...and this is why: The Cloud works so damn well, that it doesn't appear to matter what glass you put between your mouth and the Cloud, it seems to get the meds to ya in a very reasonable fashion!
I've vaped with my Cloud, using several pieces, but nothing super high-end, with the exception of a Sheldon Black straight tube with showerhead perc. Otherwise it runs through my Evolver HD 2.0, and my two FC bubblers most commonly...and in the mornings, I almost exclusively use the dry mouthpiece now, with the temp anywhere from 7am-10am...Heck, I've even thrown a couple of different oil domes on the Cloud to hit it with practically no mouthpiece...not the smoothest/coolest hit ever! hahaha! Would have been a better idea at the 7-9 range on the temp dial than the higher temps that I used.
Anyway, the point of this post is that the Cloud, IMO seems to do a well enough job at what it does that it doesn't matter what glass you use with it! I will say though that I have never had a subpar experience with the three Hydracircs that I have tried (my first, that broke on my part...my replacement, and another members here.) and if you can't afford anything else/are indecisive, I'd go with any of the Hydratubes. IMO, some lower end glass will have the potential to be less satisfying to use than a HT for the average user...IMO.
So, if in doubt, go for a HT, or use with your existing collection....but in most cases, I don't think you could pick a bad piece...Just keep clearance in mind, and purchase an angled 18mm adapter