It all depends on what you want to play really. If you’re into more traditional forms of metal like thrash, heavy metal, power metal, or hair metal then a six-string instrument will likely do the job for you. You can tune it down if you want to get heavier, but it’s not a must-have as you’ll likely be playing closer to standard tuning.
If you’re into more modern metalcore, djent, or progressive metal, then an extended-range 7-string guitar or 8-string guitar is definitely the way to go. It gives you that super low end and these guitars usually feature longer scale lengths to deal with lower tunings. You also get the ‘regular’ range of a guitar, but it will take some getting used to thanks to the extra string.
If you’d like to keep the playability of a regular six-string but still want to tune low, then you should have a look at the best baritone guitars. These guitars feature a longer scale but keep the six strings, so you can still play open chords as you would on a regular guitar, but in a much lower tuning.