The intro to Manowar's Bridge of Death is a pretty well-known case, and if you're excluding early thrash from extreme metal, Slayer's Fight Till Death and Black Magic might be the earliest examples.

Aside from the non-extreme bands influenced by black metal or post-rock (tremolo picking is as much a trademark of post-rock as it is of black metal) like Agalloch, I can only remember it being implemented in guitar solos.

Venom did it way much earlier with songs like Witching Hour and Black Metal. Also Judas Priest could be considered as pioneer with songs like Exciter, dating back to 1977.

It's the last riff of Strid's End of Life, which has also "inspired" Nargaroth on Black Metal ist Krieg.

I'm not sure Exciter counts as trem picking. The main riff is just 16th notes at a moderate tempo. Trem picking blurs the rhythm of the individual strums a bit.

Forefather - Steadfast - Has that intricate, stately atmosphere coupled with a rockish drive the opening riff is what I'm referring to:https://www.youtube.com/watch?v=Uytm9-C-Nl8

Iniquity - Border into shadow - Another 'sound defining' riff for Iniquity: A grimy and sister form of crunchy tech death, the melody is impossible to forget and it starts at 0:34 - https://www.youtube.com/watch?v=LoG56iIUkKE

Dodsferd has some pretty damn awesome pissed off riffs like in Hypocrtic Shitfuckers still breathing. Like just look at the title of the song lol.

Also Forteresse's Honneur et Tradition is absolutely amazing. I noticed how that was recommended so I definitely second it.

L'Espirit du Courant by Brume D'Automne is pretty damn crazy too.

Oh and I will lay down my bones among the rocks and roots by Wolves in the Throne Room. Pretty much the whole song is just majesty. The last part is amazing. Prettty much any WITTR song has amazing tremolo riffs

It was used in surf music. The first example of this I ever heard is "Misirlou" by The Surfaris from the early 1960's; and for me, this remains the classic example. This song--probably more than any other--is what got me interested in instrumental hard rock.

Here is a more recent performance:

The Surfaris - Misirlou

Dick Dale is credited with popularizing this old Greek folk song as an electric guitar classic; though I don't think he was as well known in Minnesota, which is where I was at the time. Here is an example of his work:

Miserlou - Dick Dale

The intro riff to Vader's Wings boils the blood, but I love how the Elegy by Impetuous Ritual almost sounds slower than it actually is.

The opening riff on Altar of Plague's 'Neptune is Dead' is my favourite. I can't describe why I like it so much, I just do.

Possessed - The Exorcist

Awesomeness starts at 1:52.