+1 on Pygmy Corys - I have a shoal of 8 and they are by far the most active and fun fish in my tank. The shoal up and swim all round my tank at all levels and really liven it up.
I also love the other 2 dwarf cory species; habrosus and hastatus
<------ My favourite fish out of what I've ever kept was my dwarf gourami, he was rather fun. I don't think they're my favourite ever fish...I don't think I have come across that yet.