For our clients, we use Google Analytics. It's free and we can fix their funnels, design experiments and evaluate the results. We really only use Google Analytics and occasionally landing page software, like Unbounce.
We went for Nutshell because we are just looking for simple. When I was doing all my research, I reached the conclusion that CRMs are generally not good. I don't really like any of them. If I had time and space, I would have my team build one, but for now we went with Nutshell because it's so lightweight. I like that it integrates inline with emails; as long as you associate your emails with leads, you can figure out the last correspondence with someone at your company.
Slack is pretty good. We still wind up using Google Hangouts a bit, and we have designers talking on appear.in. Slack is keeping track of everything, which is great, but if the job that it's supposed to be doing is keeping everything in one place, it doesn't. We just started paying for it, and I'm not sure I'm getting full value from it yet.
I'm relatively new to consulting, so I wanted my project management tool to be good for my clients. Basecamp has great client access management, which is why I picked it. Its actual PM tools work just fine, but the client access is what matters to me.