I will be flying from Phuket on Thai Air to BKK to catch a midnight flight on Korean. I am pretty sure I will have to claim checked bags as the two flights are booked separately.
I can arrive at BKK at 5:50 or 8:35. It will be on a Friday night in summer.
Which should I book to ensure I have enough time to get bag and go through immigration?

So here's your itinerary:
HKT-BKK arrives at either 5:50/8:35
BKK-XXX leaving at midnight

Assuming that you did the work, i'm guessing your 5:50/8:35 is pm, in that case on a Friday night I would choose the 5:50 to make sure there's plenty of time to grab food/drinks then go to check your baggage for the second flight.

Either would be fine as you have plenty of time even if the 20:35 TG flight arriving a little late. It really depends on where you want to spend those extra hours.

I'd personally go for an early dinner by the beach in Phuket and then leave for the airport at 18:30. You have plenty of time to check in for the KAL without any rush.

With the earlier TG arrival option at 17:50, I'd probably do an early check in (assuming that KAL are open for check in) and then take the Airport Line into town for some dinner for a couple of hours. Then back to the airport on the ARL by 22:30 at the latest. Immigration won't be too bad at that time especially the east zone....west zone is usually busier from around 23:00 onwards......

Take the later flight to BKK. The KE check-in counter does not open until at least three hours before departure. If you get to the airport early, you will wind up sitting in the departures hall. It's crowded, boring and loud. Trust me, I made that mistake once. Even if it takes you an hour and a half to get through immigration and get your bag, which it never does, you will still have two hours to check-in and go back through security. If you are at all cutting it close - which you won't be - ask the KE agent to give you a pass for the faster security line. It's technically for elites and airline personnel, but if you are cutting it close, they'll likely give you the card.

FYI � The flight from HKT-BKK is internal, so there will be no need to clear immigration upon arrival. I would guess about 30 to 45 minutes from deplane to walking out into the arrival area max.

Most airlines don�t open their check-in much before 3 hours prior to departure, so if you catch the earlier flight you�ll be left hanging around with your bags until check-in opens and then a further 3 hours until departure. Boring

Although you said that your flights are on separate tickets, you could ask the check-in desk clerk at HKT if it would be possible to have your bags checked through to your next destination. It will depend if TG has an interline agreement with KAL and the mood of the clerk/supervisor. Nothing to lose, so worth asking.

If you decide to take the earlier flight and have to collect your bags there is a left luggage facility at the airport if you want to spend a few hours in town.

Personally, I would take the later flight. You will have plenty of time to collect your bags (if necessary), check-in and clear immigration without any rush.

D'oh. My bad.

Which I've done, for that flight, and can attest there's nothing fun about sitting on a bench with strangers watching KE employees slowly set up for the night. If you can get your bags checked through and do not have to leave the secure area, there are shops and restaurants to pass the time. There's nothing but a very jammed, crowded convenience type store before security.

It looks like there may be such an agreement. Link. (Note, the KE link on the page is unhelpful.)

Thank you for this. I have been googling and could not find it.