In addition to LHR, my usual go-to's are CLT-FRA/MUC, JFK-MAD/BCN/MXP.
While I've never not cleared the upgrade eventually, I've never had much luck finding space in advance (even off peak) to anything but LHR from DFW or MIA, like MAD, CDG, AMS, etc. DFW-FRA will have a small bit of space every now and then.