>>19250045
Well... it depends on how much you want to stay in the 40k universe
If you absolutely must, Dark Heresy is the easiest one, not because of complexity (it's about on the level with the others, but the rules are a bit dated), but because it's not horribly bloated; in Deathwatch, for example, a creature will have a statblock that looks like pic related, and the characters are even more filled with stuff that you can't just tell what does.
If you're willing to try something else, THE best entry-point in my experience is Lady Blackbird
http://www.onesevendesign.com/ladyblackbird/
It's free, small, simple and, most importantly, you don't need any other preparation than printing it out and giving it a once-over.
So yeah, it depends. I started with Dark Heresy, and looking back on it it wasn't an ideal entry-point either