The best is between your ears and all the different browsers if you have bothered to build your site properly. More than 99% of the available tools are useless and are a more useful tool for situations where you are working hard and you are designing specific resolutions as a fixed-width metric or a fixed-character metric instead of being a good little doobie and build a semi-fluid responsive elastic design.
Design the office layout first as follows:
1) Elastic, to the extent possible, fonts, widths, padding and margins are declared in EM.
2) semi-fluid. At least one column is allowed to span the available space and contract below. No dumbass px trash of fixed width.
3) Reactivate, delete the columns and complete the queries with the media when the screen becomes smaller.
Make your desktop layout as a semi-fluid elastic. Minimize the browser window until it breaks, you will find the stop point of your multimedia query. Determine the number of EMs, add 5% more leeway than font rendering, delete / rearrange columns, eliminate excess whites, and rearrange other elements until you get an aesthetic look. Clean, rinse and repeat until you get below 256px with the default browser resolution of 16px … or 320px if you are running large fonts / 20px … or whatever the metric of your system font.
If you think in pixels, thinking about specific widths, etc., etc. You are all wrong, and overall, if you think of specific devices or resolutions, you can also give up making the site accessible.
Some automated tools can not report or even solve this problem.
Oh, and @alfieindesigns, the mentally weakened train wreck on NOT building websites is the last thing I suggest pointing them at. Learn HTML and CSS, not a stupid framework that only creates inaccessible sites that are broken, slow, and inflated. In particular, when it is painfully obvious even that the people who created bootstrap are not qualified to write a single line of HTML code … just look at their examples. There's nothing like fooling people with outright scams while getting you to work ten times as much as you need with two to twenty times the work you need … and calling it "easier"