John O' Groats is a shithole.....I wouldn't waste any significant time getting to it as it will be a big disappointment (although some of the roads nearby are great).
The usual advice is "West is best", athough you have a decent amount of time, so imagine you could take in as much as possible.
Here's the usual "west coast loop":
http://maps.google.com/maps?f=d&source=s_d&saddr=Luss,+Alexandria,+Argyll+and+Bute+G83,+UK&daddr=Tyndrum+to:Ballachulish,+UK+to
ornie,+Kyle,+United+Kingdom+to:Strathcarron,+United+Kingdom+to:Applecross,+Strathcarron,+UK+to:Fearnmore+to:Kinlochewe,+Achnasheen,+United+Kingdom+to
rumchork,+United+Kingdom+to:ullapool+to:Inverness,+United+Kingdom+to:fort+augustus&hl=en&geocode=FW8IWAMdATi5_ykJbMGVu6qJSDGGxGnWEjgOeg%3BFSEgXQMddSW4_ymlLnMfth-JSDHQzN93HGgMCg%3BFU3CYAMdD6-x_ykTWC0nqyiJSDHkSafKJGERuQ%3BFdkCagMdYtur_ylTyBXLBJGOSDF29IgLb3LHKg%3BFYY4bAMd0Cut_ykP7Mv4mmKOSDHNpX3nGZBTog%3BFUJabAMd_0Wn_ylNPyyoC4GOSDHtYmEBs3YSng%3BFVORbgMdUlmn_ympYV_8vX6OSDHAdt93HGgMCg%3BFav5bgMdqhOv_ymn2o9LuV6OSDGnXvmXOk3Sag%3BFcd_cgMdpNyq_yn3K47EfmuOSDHw9-z_V9Qd3w%3BFVJ6cwMdu0yx_ylhs6e3-zCOSDENyMkBc5PEWQ%3BFVcGbQMd4m6__ynh4a9rHsKOSDHgFuch3CDaLQ%3BFUvvZwMdMou4_ynrOxv0DSGPSDHQPK8hlw8XEg&mra=ls&sll=57.000395,-5.039305&sspn=1.893752,6.696167&ie=UTF8&ll=57.049694,-4.487915&spn=1.891316,6.696167&z=8
It has a lot of good places to see and a great drive:
Starting on Loch Lomond side take the A82 all the way through Glencoe - absolutely brilliant drive.....stunning scenery, mostly a decent paced section (i.e. not stuck behind caravans for most of it).
Follow the A82 towards Invergarry, then take the A87 to Dornie and Eilean Donan Castle - again, great great roads and scenery and again decent roads for overtaking/pace.
From here head up towards Strathcarron, make sure you have a full tank of fuel as this is where it gets a bit "wildernessy". At strathcarron you begin the famous Applecross run round the coast - lots of single track, but very quiet and scenery you won't believe (most people don't realise Scotland is quite this beautiful).
Follow round to Kinlochewe, at which point if you're fed up you can head straight to ullapool, or go for round 2 on the single track for more epic scenery.
Either route brings you to Ullapool - decent spot to stop and recharge for a day or so.
Heading back down through Inverness, down the side of Loch Ness, a stop in Fort Augustus (beautiful town) for lunch.
I'll try get some photos and more details of the route later.
From this route if you want to extend it there are some obvious detours:
From the start at loch lomond you could opt to go through Inveraray and on to Oban, but doing so would mean you miss Glencoe, I'd favour Glencoe, but if you had the time to loop back its all great roads.
At Eilean Donan castle you could skip over to Skye - never been myself, but people say its great.
Once at Ullapool there are some fantastic roads to the north, and you could make your way over towards John O'Groats via Loch Assynt - on the way is a tiny place called Kylescu and the Kylescu Hotel which serves great seafood (so I'm told, don't touch the stuff myself, but its certainly fresh as you see the guy get on his wetsuit and go fetch it from the loch).
From there you could follow the very northern road through Durness and "over the top" to the east.....no a place I've been, so can't comment.
Will come back later with some more suggestions...