Bear in mind you can visit them all if you want. The LH lounges are at the main terminal (Business lounge and Senator lounge which is within the Business lounge complex), but you'll be leaving from the B Gates so you should venture there where the other three lounges are, and the SQ and AC lounges are two sides of the same entrance, so you can visit both very easily. UA is only a few minutes walk away, but its main highlight is the tended bar with cocktails (personally find the UA food inferior to the SQ lounge). Only the SQ lounge has announcements for SQ flights, but you'll find they are each different and all equally new (they all opened within a one year period).