Only thing to watch for which catches a lot of beiginners out is that some grade I gullies can have massive cornices to deal with which are either vertical at best, or sometimes overhanging. A story on UKClimbing a few years back about a chap who was trying to get grade I gullies under his belt for his MIC I think and went up Tower Gully and ended up hacking at the cornice for 5 hours or something. Tower Gully is also a grade I.
I would say Broad Gully, Stob coire nan Lochan is the easiest. It is relatively easy angled and poses no cornice issue at the top. It is a good introductory gully, just watch out for climbers bum sliding down. You can see me going down Broad Gully at the end of this video:
Next grade I options are as said Gully 3 and 4 on The Ben, but beware as said about cornices, particularly with no 3 Gully which is steeper at the top. There may or may not be notches cut in the cornices already.
No 3 Gully with climber approaching cornice.
http://www.ukclimbing.com/images/dbpage.html?id=189711Of all the grade I gullies I think Tower Gully is the most imposing and scary of them all. You are on steep grade I ground all the way up Observatory gully on the flank of Tower Ridge before you even get to the actual foot of the gully which is above some cliffs.
Tower Gully traverses in from the bottom left above the icefall and cliffs, to where the top set of climbers are and then following that left trending slope.
http://www.ukclimbing.com/images/dbpage.html?id=81596