Best-North Korea by far. Immigration officers and customs very friendly. Much more so than their Southern counterparts, but I guess they aren't as busy.
Worst-Belarus. No one comes close. They openly asked for a bribe to let me through quicker on my way out of the country. I declined, saying I'd rather wait. I was one of 17 passengers, the rest a mix of Belarussian, Russian and Lithuanian businessmen on our way to VNO. They clearly had no luck with the "locals" so when they saw my passport their eyes lit up. I could only imagine what was going through their heads, the things they were thinking of charging me for. They were really struggling to find anything at all in my bags they could try and extort "duty" out of as I had bought nothing in Minsk other than a few postcards and a fridge magnet. They were taking their sweet time, obviously trying to scare me into paying something, knowing my flight was leaving soon. I didn't flinch at all and in the end they got pissed off looking through my dirty clothes and let me go.
Must say I found Aussie customs to be quite pleasant, I wasn't even asked anything, but I was inspected for Quarantine (muddy shoes) and they were also pretty friendly too.