I have always considered STACS, the Symposium on Theoretical Aspects of Computer Science, the best venue for computational complexity in Europe. I have attended the conference many times and they consistently have several strong papers in the area as well a good attendance of complexity theorists from both Europe and America. You can see the weight complexity gets on the web page where "Computational and structural complexity" gets the same weight as "Algorithms and data structures, including: parallel and distributed algorithms, computational geometry, cryptography, algorithmic learning theory".
The ICALP conference has a longer history, a larger audience, more traditions and does a better job representing Europe as a whole. But the scope in ICALP is quite large and computational complexity often gets lost in the shuffle.