We're going to be headed back home after a stay in Berlin. We're trying to figure out which airport would be easiest to get through to connect to the longer flight from London back to Los Angeles.

There seems to be 2 major airports in Berlin...which to go through?

You probably want to choose that airport which serves the one in London from which you will depart to L.A.

The return will probably go from LHR to LAX - just looking at how to get to Heathrow with the least amount of trouble.

I don't believe there's non-stop flights between THF (Tempelhof) and LHR. So, your only choice is TXL (Tegel).

No SXF (Schoenefeld) - LHR non-stop either.

You're going back the day before, right? In that case, it does not matter so much. Just take whichever airline has the best time and cost, spend your last night in London, and head out to Heathrow the next morning.

I would most likely opt for TXL to LHR (provided the transatlantic flight departs from LHR and not from LGW).
TXL is easily reachable by bus. On the downside only BA serves LHR from TXL (allow for some delay as BA has an appalling on-time record from my experience). LH serves LCY nonstop only, which is on the other side of London and makes for a really long commute to LHR.

THF is not an option for flights to London as far as I am aware. It's also supposed to be closed down (despite or because of its great location right in the city).

SXF is mainly catered to by low-cost carriers or so-called low-cost carriers (Air Berlin etc). Hate them for lack of flexibility and strict check-in times. It doesn't help that SXF is such a long way out in the boonies.

Thanks everyone - currently we're trying to find the best way to use up miles for upgrades on the longer legs.