In fact, Inmotion is quite good. However, I don't think the best host even exists. Everyone has different needs. So, what's best for you might not even be a good solution for me.
As Inmotion is using more bandwidth providers for their business plans it is possible that the personal plans are a little bit slower.
How much traffic can you get? Well, that's a difficult question.
In their ToS you find the following:
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Unlimited Disk Space & Bandwidth
While IMH does not meter disk space & bandwidth, the purpose of an IMH hosting account is to host web sites. Using a hosting account primarily for online file storage or archiving electronic files is prohibited.
IMH Personal & Business Class hosting are shared hosting environments, so to ensure fast & reliable service to all of our clients, accounts that adversely affect server or network performance must correct these issues or will be asked to upgrade to a virtual or dedicated server.
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In conclusion: If your website affects the performance of other sites on the server, they will ask you to upgrade, which is quite common.
Unfortunately, you can't know how much bandwidth you could possible use without affecting other sites. This depends on a lot of factors: How many sites are on the same server? What are the specs of the server? How much CPU / RAM / database calls does your website use? How big are your files? How many visitors does your site get? Etc.
You can only try to build a rather lean website.
Personally, I recommend E2 web hosting. They offer great hosting for a very affordable price. I was able to serve a huge amount of bandwidth with a well coded site hosted at E2.
Here's a link to check them out: http://www.unlimband.com/webhost/e2.php
I host most of my sites with them and I'm very satisfied with their service.
Hope this helps.