"Best" depends on your future needs, or if this is a one-off situation I suppose. This question comes up somewhat regularly, generally in the Schedules forum which is where I'm moving this - as the issue is more a scheduling issue than a modeling one.
First, I would suggest that you verify if the manufacturer honestly would ship the 10-sash window you're showing as a single mulled unit - vs a 2 + 3 etc.
You can fake this with the W Ribbon 20 window, set to show 5 sashes, and select "Custom Sash" as the sash style, creating your own custom sash that models the bottom awning/hopper (I assume that's what it is in the image). You'll have to fake the opening lines in the schedule and if you want them to show in elevation.
You can also build your window by mulling individual windows, convert it all to a Morph, then convert that back to a window object (after laying it flat on the ground) as another way to 'fake it' for a one-off. But, it won't respond to model view option control for display of opening lines etc - you'd have to fake that.
If you want 100% control over which sash are operable and how, and for this to appear as a single unit, and to be a live model element - then Cadimage D/W is the way to go.
GS UK developed the Modular Joinery object which can probably do what you want, but it isn't available outside the UK as far as I know:
http://archicad-talk.graphisoft.com/viewtopic.php?p=265127
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