Interesting stats for discussion (though, IMHO they should be considered with care since it seems that Spanish meta is completely different from the US one).
Based on 8 tournaments with 161 total participants (averaging to 20 participants per tournament)
Win percent rate of Houses:
Stark 57%
Martell 53%
Baratheon 52%
Targayen 52%
Lannister 51%
Greyjoy 45%
Win percent rate of Agendas (Agendas not mentioned didn't have enough players, such as White Book/City of Shadows/Night's Watch was played 1 time in 8 tourneys):
Wildings3 69%
Maesters 66%
TPBTT 58%
No agenda 50%
KOTHH 49%
KOTR 49%
Siege 49%
Summer 43%
Winter 40%
PS. High percent of wins for Wildlings is probably related to the fact that it was played by a player who got in top 8/top 4 in most if not all of the tournaments he played independent of a deck.
PPS. TPBTT seems to have different versions with Aioria's one averaging to something like 75% win in 3-4 tourneys (which is mentioned as TPBTT on reports), while the rest of TPBTT decks is spread over averaging down to 58%.
PPPS. And in the end, Maesters are the best agenda even after averaging over about 21 different decks/players. At least for Spanish meta.