The easiest level to pick up & play (apart from the first, obviously) is Sink The Shipment. It's a lot of fun, has some stealth kills, is not too threatening if you're not in the mood for a real challenge, & doesn't take long to complete. It's not the most interesting or compelling level, but it probably is the one I've played the most times over.
That aside, I enjoyed The Boss Of This City, Come To MOTHER, Sacrifice, Dead Man Walking, & Death In The Family.
I absolutely hated Prey Of The Clan, 10,000 Ways To Die, If It Bleeds..., Bug Hunt, & Hot Time In The Old Town