There is no "best bot" on Steemit. First of all these upvote bots are a tool to promote your posts. The profit shouldn't be your main goal. But I need to admit that it is the only goal for most Steemians.

You could have a look at https://steembottracker.com where you can find a list of various bots together with their min and max ROI. Some of them promise profit others don't.

Maybe you search for something like Minnowbooster or Smartmarket ? They promise profit but you cannot buy very high upvotes there. Just for a few dollars.
Check them out here:

• http://minnowbooster.com
• https://smartsteem.com/buy