Which are the easiest models to paint in Warhammer 40k?

I have been really getting into warhammer 40k but I haven't made any effort to try and paint my own. I've been buying used models already painted. I really want to start painting but I don't know where to start. What are the easiest models to paint for a beginner like me?

Any army is cool, I already have a huge collection so I am not limited to any one thing.

Thanks guys!Which are the easiest models to paint in Warhammer 40k?
Orks. there a rather crude race, so if you're not to good at painting it's ok.Which are the easiest models to paint in Warhammer 40k?
Orks, Tyranids, or Space Marines, shove as many helmets as possible in the space marines and you got basically a wave of one color(with the exception of apothecaries and chaplains of course)Which are the easiest models to paint in Warhammer 40k?
orks and tyrannids are easy and usually look good regardless of the color scheme