Well, from an empirical point of view, there was a recent study in the Journal of Strength and Conditioning Research in Oct 2012 which actually measured which was the best exercise for shoulder stimulation. They hooked up electrodes to the shoulder muscles and measured the electrical activity that was stimulated in response to each exercise. Dumbbell standing presses created the highest EMG reading. Barbell standing presses was second. The authors attributed this finding to theory that because dumbbell standing presses were the least stable or in other words required the most stabilization, it therefore created the most electrical stimulation. However, the 1 RM was lower than standing barbell presses, but thats ok, it just means you don't have to try and use as much weight to get more stimulation.
Hope this helps. Seems a lot of people here are already incorporating the most electrically stimulating shoulder exercises.
Here is the abstract:
Effects of body position and loading modality on muscle activity and strength in shoulder presses.
Saeterbakken AH, Fimland MS.
Source
1Faculty of Teacher Education and Sport, Sogn og Fjordane University College, Norway. 2Department of Public Health and General Practice, Faculty of Medicine, Norwegian University of Science and Technology, Trondheim, Norway. 3Hysnes Rehabilitation Center, St. Olavs University Hospital, Trondheim, Norway.
Abstract
ABSTRACT: Little is known about the effect of performing upper-body resistance exercises with dumbbells versus barbells and standing versus seated. Therefore, this study sought to compare electromyogram activity (EMG) and one-repetition maximum (1-RM) in barbell and dumbbell shoulder presses performed seated and standing. 15 healthy men volunteered for 1-RM and EMG testing with a load corresponding to 80% of the 1-RM. EMG was measured in the anterior, medial and posterior deltoids, and biceps and triceps brachii. The following EMG differences or trends were observed: For deltoid anterior: ∼11% lower for seated barbell versus dumbbell (P=0.038), ∼15% lower in standing barbell versus dumbbell (P<0.001), ∼8% lower for seated versus standing dumbbells (P=0.070); For medial deltoid, ∼7% lower for standing barbell versus dumbbells (P=0.050), ∼7% lower for seated versus standing barbell (P=0.062), 15% lower for seated versus standing dumbbell (P=0.008); For posterior deltoid: ∼25% lower for seated versus standing barbell (P<0.001), ∼24% lower for seated versus standing dumbbells (P=0.002); For biceps, ∼33% greater for seated barbell versus dumbbells (P=0.002), 16% greater for standing barbell versus dumbbell (P=0.074), ∼23% lower for seated versus standing dumbbells (P<0.001); For triceps, ∼39% greater for standing barbell versus dumbbells (P<0.001), ∼20% lower for seated versus standing barbell (P=0.094). 1-RM strength for standing dumbbells was ∼7% lower than standing barbell (P=0.002) and ∼10% lower than seated dumbbells (P<0.001). In conclusion, the exercise with the greatest stability requirement (standing and dumbbells) demonstrated the highest neuromuscular activity of the deltoid muscles, although this was the exercise with the lowest 1-RM strength.