In reply to Lifeismeaningless:
If you really do need to restrict yourself to staying south of the Great Glen, you are missing out on possibly the most spectacular Munros of Torridon and similar, such as An Teallach, and of course the Cuillin on Skye.
But, staying south of the Great Glen, Glen Coe would be the first port of call, and in particular, Buachaille Etive Mor, the two Munros of the Aonach Eagach ridge, and Bidean Nam Bian. Buachaille Etive Beag is less spectacular, but only really because its neighbours are so special.
In the same part of the country is Beinn a'Bheithir, with two Munros. A little further south is Ben Starav, and its satellites, Stob Gabhar, and Ben Lui.
Ben Cruachan would also be on my list.
And then of course you have the Cairngorms ...
But don't restrict yourself to Munros. Smaller mountains are no less worthy, and in particular the Cobbler is a fine peak with a nicely satisfying tweak at the top.
And for a much less busy day out, the Merrick, in Dumfries and Galloway is good value.