All three major AI assistants can write emails and summarize documents reasonably well now, so the useful question isn't 'which is smartest' — it's which fits how you actually work.
ChatGPT
The broadest plugin and integration ecosystem, and the most third-party tools built around it. A solid default if you want the widest range of use cases covered by one subscription.
Claude
Tends to handle long documents and nuanced writing tasks particularly well, and is a strong pick if a large share of your work involves reading through or drafting long-form text.
Gemini
Its advantage is deep integration with Google's own tools — if your business already runs on Gmail, Docs, and Sheets, the friction of switching between your AI assistant and your existing workspace drops to near zero.
How to actually decide
Don't subscribe to all three out of indecision. Pick based on your existing toolchain (Google Workspace leans Gemini, heavy document work leans Claude, broad general use leans ChatGPT), try it for a real work month, and switch only if you hit a specific, repeated limitation.