Formal:
* Extensive vocabulary
* Rich vocabulary
* Comprehensive vocabulary
* Broad vocabulary
* Wide-ranging vocabulary
* Well-developed vocabulary
Informal:
* Knows a lot of words
* Has a big vocabulary
* Words galore
* Wordy (this can be used humorously, or to imply someone uses unnecessarily complex language)
More specific:
* Specialized vocabulary (for a specific field or topic)
* Advanced vocabulary (indicating complex or sophisticated language)
* Fluent vocabulary (meaning words are used naturally and effortlessly)
The best alternative depends on the specific situation and the tone you want to convey.