Any vacuform kit. They have always defeated me.
Most challenging kit from a painting perspective for me has been the Polar Lights 1/350 scale refit USS Enterprise NCC-1701. Achieving the subtle "Aztec" pattern is not for the faint of heart. Aftermarket mask sets are a must.
Concur with praise for Tamiya's P-47. It's a gem. Every more recent 1/48 scale Tamiya kit I have owned is really well engineered making for easy, painless assembly.