I read this question as "What is your favourite type of Deep Sky Object"
for observing because that's what I do at my scope, I don't capture images, etc.
If I had interpreted the "thrust" of the question differently, ie "which DSO's fascinate me the most" my answer would have likewise been different.
I was going to put nebula originally but realised that apart from a very small number (Orion, Carina, etc) I much prefer to look at magnificent images of them so I can see them revealed with all their beauty of form and colour displayed.
I have chosen Open Clusters for much the same reasons as Rod listed, except that he worded his explanation better than I would have.
I do want to add that I am a BIG fan of observing Globular Clusters - I love them!