Determining the "best" tool really is a part of determining what your needs are. The different tools out there are all pretty good, but they do things differently and delivery different things. You need to understand what you want out of the software and that will make choosing the best one for you easier.

Full disclosure, I work for Red Gate software.

I would split the tool sets into full-featured, enterprise-level monitoring, and small to mid-range monitoring. On the high end, I can't recommend SQL Sentry enough. Great piece of software written and supported by great people. But, it's expensive and if you're in a small shop, it might be massive overkill. For small shops with more limited needs, I recommend you take a look at Red Gate's SQL Monitor[/url]. It's a good piece of software. You can download it and install it for free and check it out for a few days, see if it does what you want.