The worst Immigration experience was in Cambodia. About three years ago another family joined us from Colombo to Phonm_Penh. We cleared immigration first , took less than 3 mins and was waiting till the other family joined. Waited over an hour, no signs of them, I cannot go inside again but I had a Cambodian Driver/Guide with me. After one and a half hours he went inside. He was questioned how he knows them. He really didn't know them as all his dealings were with me. Finally after further checking them, an a body search too, mainly as harassment,, they let them in.
Afterwards I found out that all they required was just $10USD enclosed in the passport.
So if anyone going there please note, even though you have a valid visa, a tip goes a long way.
On the service side the worse is Santiago (Chile). Santiago is very busy, as it's the central point in South Americas. We flew in from Punta-Arenas to Santiago and had one and a half hours to check in for the next flight to Lima, Peru - same Airline LATAM.
The queue was horrible, only 4 or 5 counters manned, they are very slow. Aftre waiting we jumped the queue saying to others that we will miss the flight. Finally when we arrived at the gate, gate was closed, the aircraft doors were open, but not one single airport or airline staff to let us in through the gate. We missed the flight. (they unloaded the baggages)
Later on when complained to the airline, they sent a staff member, We had 7 in the group. I was taken out again through the immigration, taken to the airline office, given tickets to the next flight which was due in 3 hours. Then the same girl came with me and escorted to the gate back through the immigration bypassing the queue. Luckily we arrived nearly midnight in Lima. We had to take another flight to the Amazon, the next day at 9AM. If anyone transiting through Santiago make sure at least 2.30 hours to the next flight.
Also there is a bit of a delay in Swarnabhoomi (Bangkok) if you transit, with the security checks. They send all the transit passengers through a bottle-necked security check. But it's not that bad, but has a considerable delay.
I have traveled to many countries, except to Russia & the two Koreas, and the above ones are the worst experiences.