OpenAI ChatGPT 5.5 is the newest flagship (GPT-5.5) with a 1,050,000-token context window and up to 128,000 tokens of output. Better reasoning, creative writing, coding, and multimodal understanding than 5.4. Best for very long documents, large codebases, and long-form generation. For everyday tasks, 5.4 mini or nano is cheaper.