Great Weapon Master locks you into a specific weapon and style. I don't like it especially since it forces Reckless Attack instead of making it optional. Additionally, if you look at the Barbarian's class features, it's the best class for grappling and shoving thanks to Advantage and later a Better-Than-Reliable-Talent on Athletics checks. You can't grapple while wielding a two-handed weapon.
Reckless attack is, to me, a sometimes feature, not an always-on. Surrounded by 4 enemies? My Totem Barb won't reckless attack unless they have high ACs that he can't consistently hit otherwise. With GWM, you have to use Reckless to counteract the -5, leading to more damage taken.
I favor two weapon fighting. The extra attack adds substantial extra damage in most rounds, and if you want to grapple an enemy, you can still wield your primary weapon, making it a seamless transition. Polearm Master with a spear also works since it's versatile, but the BA attack is a bit weaker and it feels like it should be paired with a shield, which then locks out easy grappling.
Caveats:
*You want the Dual Wielder feat ASAP so you're not stuck with shortswords. It's also worth +1 AC. Half plate, Dex 14, and Dual Wielder is AC 18, which isn't bad.
*A Fighting Style feat for Two Weapon style gets you +Str to your off-hand attack. You want it eventually, but by my math, it's not worth getting until your strength is 20. 18->20 gets you +1 to 3 attacks per round and +1 to 2 attacks per round, while the style gets you +4 damage to one attack per round. So you pick that up at like 12.
*You don't get your BA attack on the round when you rage. It's also incompatible with BA-using subclasses such as Storm Herald and Wild Magic.
Beast Barbarian also gets you an extra attack, but I don't like being locked into natural weapons that never really improve in a world where Frostbrands, Flametongues, Blackrazors, etc. exist.
GWM Barb at 19: Str 20, 2 hits for 2d6+5(str)+4(rage)+10(feat) = 4d6+38 = 52 DPR
TWF Barb at 19: Str 20, 3 hits for 1d8+5(str)+4(rage) = 3d8+27 = 40.5 DPR
The more magic items you have, the better dual wielding gets since it benefits more often from anything that boosts your damage, like belts of Fire Giant strength, +xd6 weapons, etc. If magic items boost each attack by another 9 damage (+2 str modifier, +2d6), then the gap between the GWM and the TWF closes by that amount.
However, although GWM's damage is better, there is still more of a risk of not hitting, even when Reckless Attacking. That counts for something against GWM's value, but how much depends on the enemy's AC.
GWM is still a bit better for damage, but you're locked into Reckless a lot more often, which can work against your "I don't die" shtick. To me, the Fighter deals more damage, but the Barbarian has better battlefield presence in terms of immobilizing opponents with grapples and simply being unkillable. TWF supports that better.
As far as Subclass, Totem can get you a lot of damage resistance, more skills, and limited flight; Zealot gets you the most damage. You'll be happy with either one.
