Claude Sonnet 4.6. With some brief experimentation with Gemini 3 Pro and GPT 5.3 Codex (both of which seem excellent so far). Perversely, I'm more comfortable at the moment with Claude Sonnet 4.6, just because I have a ballpark sense of how far it can be pushed before it loses it, and haven't yet developed the corresponding bat-sense for Gemini 3 Pro and GPT 5.3 Codex.