It depends. If you are already into really extreme metal, then maybe you should start with SYL. They're very weird and, well, extreme, but also very melodic. So if extreme metal with good melody is what you want, SYL is a good place to start.
However, Devin Townsend is also excellent (and maybe better -- I didn't like SYL at first, and I think I'd have liked his solo stuff better) to start with. It's still metal, but it's more progressive, not so extreme, and more emotional (maybe you could argue this).
I recommend the following albums from SYL and DT to begin with:
SYL - City; pure mayhemic awesomeness, and has several of their best songs. The new one, Alien, is very close behind this one, and gets better with every listen.
DT - Terria or Accelerated Evolution; Terria is more experimental, has a lot of strong emotion, takes more time to sink in, and for that it may be better; AE is more straightforward, definitely catchier but with no less depth, has some very emotional parts -- I'd call this more a 'songs-album', whereas Terria is an 'album-album', if you know what I mean.