When it comes to the biggest names in thrash metal, Sepultura flies under the radar, but I consider them the best-ever thrash band. I guess they're an outlier of sorts, stemming from the fact that they're from Brazil, and that their vocals are harsher (more death metal-like) than your typicaly thrash.
Anyhow, I've been listening to them since the early 90's, having bought Arise when it 1st came out. When I recently put on Beneath The Remains, instead of brainlessly headbanging away, I consciously tried to listen for some of the things that originally captured my imagination about this band.
Time changes. Loads of them. Sure they're fast n' heavy as fu&k, but within a given song, this band crams in a number of stops, starts and changes worthy of a 70s prog band. It's just not quite as noticeable because they're seamlessly done at thrash metal speed. And within those changes, they cram in a plethora of the juiciest heavy riffs.
If any thrash metal band could be labelled 'experimental', then Sepultura has to be it. This is borne out by the new rhythms they tooled around with on subsequent albums Chaos AD and Roots, sometimes to the dismay of fans of Schizophrenia and Arise.