I've really enjoyed modelling my Deathwatch, simply because literally every model is a kitbash/conversion of some kind. Painting them is less fun (black is kind of... meh to paint for me), but putting them together has been fun.
In terms of most fun to paint, my Wood Elves were pretty fun. I opted for an winter theme, so it's all white/ivory and blue, with lots of wood/leather/stone accents. Eldar were also fun, but I made myself a little crosseyed painting every single bump as a gem.