BBC Sessions was the first Led Zep stuff I ever heard (other than Whole Lotta Love that is), so like with others that has a special place.
Out of HTWWW and TSRTS, I'd have to say HTWWW - I personally think that it's consistently good across all 3 discs.
With TSRTS, I found that the guys only seemed to get into it from about No Quarter onwards, the tracks prior to that just sound out to me - perhaps No Quarter was when the drugs wore off?! Haha.