For me that would be Thorpe. The only ride that I haven't ridden there would be Storm Surge. It doesn't look too horrifically bad, but it always seems to pick up a rather large queue that just makes it not worth going on. On top of that it just looks incredibly dull. I assume I am not missing out on much. Though I wish I could say I hadn't been on Tidal Wave, that ride was certainly not worth riding, it was just way too short and lacked any excitement.
At Alton the only "Thrill" ride I haven't done is Enterprise, I have done Zodiac at Thorpe and thought it was fun, but seeing as Enterprise is stuck in X-Sector, there's always something better to ride right next to it, and considering Oblivion never has a massive queue, there just never seems to be a point in riding.