I'd like to give some names, but I'd advise you to read the wiki sites as well, and don't be put off by a first or second song, because most bands did quite a lot of different things.
The two oldest and perhaps most influential:
The 3rd and the Mortal (Norway) &
The Gathering (Holland)
The Gatherings most famous (and best?) song:
Strange Machines, from their famous album
Mandylion. Impossible to not automatically headbang on this one. A classic! Check the crowd!
Check this song
Black Light District. Be patient and see how this develops.
Other work:
Broken Glass
These are not the easiest bands to get into, though. The Gathering changed did new things on about every album, and 3rd and the Mortal made both short, acoustic stuff but also longer epic and complex songs.
Autumn (Holland) started of as a doomy/dark/gothic kind of metal band. I always loved this song "
The Witch in Me", it rocked as hell, live. Now they have a different singer, more rock orientated. Still good, but different.
Nemesea (Holland). If you ever come upon the album
Mana (2004), it's a fantastic album with progressive metal elements, great vocals and melodies. High level guitar and bass playing. Later the band started writing simpler songs and I thought that was a big let down. Check this epic
stuff.
Good luck with your explorations.
Maybe more later.