If you're hunting for an utterly comprehensive synonyms list online, I obsessively turn to a handful of favorites that each do something slightly different.
For sheer breadth I bounce between Thesaurus.com and Merriam-Webster's thesaurus for entry-level options, and then slide into Power Thesaurus when I want a crowd-powered list with upvotes and usage examples. For older-school depth I use 'Roget's Thesaurus' entries on various archive sites, and for nuance I cross-check Oxford or Collins online. I also keep a tab open for Wiktionary because it shows etymology and regional notes that often explain why two synonyms don’t feel identical.
When I need precision, I look up collocations and frequency in Google Books Ngram or use one of the corpus-based viewers for COCA/BNC so I can see how often a synonym actually appears in real writing. If you like to browse visually, Visual Thesaurus is a fun way to explore related words like a mind map. I always enjoy finding that one perfect word after a few of these stops — it feels like a small victory.