What is the ideal platform for sharing files with clients?

It may not be "the" ideal platform, but kompoze.com ticks a lot of boxes. It is a songwriting collaboration site that is well-suited for file sharing, giving you the ability to set up each song with what amounts to folders for each stem, instrument or mix type. You can play any track directly from the site and easily download it directly to dropbox or google drive (and others) where it creates a kompoze folder and automatically puts it in a sub-folder for that song. You can upload multiple versions of each track and there's a discussion thread (for each version) so you can make comments in asynchronous fashion. The site is free but the fee to upload anything other than mp3s is monthly ..but dirt cheap, and I'm almost sure that if you skip months all the files stay intact. For your purposes, you would want to create "private" collaborations.

I'm pretty sure that if you only want to use it exclusively to share mixes with clients, then they wouldn't need to pay any fees. The monthly fee only applies to parties that need to upload content.

I think wetransfer is the easiest. I have an account that saves all my sent files, I can password protect things, allow clients a 24 hour login window.

Just upload, send a link to the file in an email, they click it and it downloads. Doesn’t get easier than that.

I will never use Dropbox. If somebody tries to send me something via Dropbox I ask them to resend using wetransfer. Total junk service imo.

I'm another happy Wetransfer user (it just works!), and like jeremygillespie I've given up on Dropbox given the number of problems I've had with it over the years.

We just had a thread on this earlier in the week!

Check out mixup.audio

You can collaborate with others remotely. Built-in notes. Ability to show clients alternate versions. Plus, there's a free version that most of the full capability, it just allows the track to be there for 30 days max. And at $10/mo it's cheap enough to work on an album project for a while and not have a major commitment.

Dropbox is great (on my Macs)
Never had any problems with thousands of uploaded mixes / stems / videos / film edits / and extensive sharing with many clients.
No problems reported from anyone, or any company / organization I have worked with.

Dropbox is smoothest imo.

Same here. Dropbox really drives me nuts.  

Sounds like there's no need to be concerned about, or value the ability of a client to make comments that are attached to a particular version. This is not meant as a criticism - just an observation. I guess mix-ups are rare - E.g. where a client suggests an attenuation at a specific point or an eq adjustment, but is referencing the wrong version.

Yes, just copy link if you want to send somebody a file and that file only to that person(s).

I've got no experience with 'collaborative' cloud sites. When I did BOC's "lockdown" LP, we just sent .wav overdub tracks to a central location and added .wav overdubs from others into my own sessions before compiling the master session for mix.

I know one thing for sure - it ain’t Dropbox. I cringe when I see someone send stuff with that.

That's actually part of what I like. I don't like things just handing out there somewhere the way Dropbox does it. 

What issues do you guys see with Dropbox? Maybe it's features that I don't use or it's worse on PC than Mac (I'm mac based and 99% of my colleagues are as well)? For me it's the simplest ever and never crashes or bugs, files are staying until I remove them (unlike wetranfer), I can use it both for sharing (download only) and collaborating (people can edit and update the files or add new files in shared folders), plus use as permanent auto updating cloud storage across all my computers.