My opinion is based partially on my experiences that really have nothing to do with the actual genera.
My grammostola escaped in my parents' car as they were helping me move out of my college house (I'm still mot sure if it was an inside job or not.)
My first and only brachypelma was a gift from family on my wife's side.
I will say though that my B smithi (hamorii) was always on display. My G rosea (if thats what it really was... it was from a pet shop and I was in college. What can I say?) burrowed and didn't show itself much.
If you put a gun to my head, I'll say Grammy. I'm somewhat biased because my first two Ts (G. pulchripes and pulchra) were so much fun, and I raised them both from little slings. I want another pulchra and an iherengi eventually. Those 3 species are amazing to me.
That said, I do enjoy the brachys too...got three of em and will likely get more. I wish they got a little bigger, but overall they check a lot of the boxes I care about.
For me the Brachypelma genus wins by default, my grammostolas are still slings and have not shown much in individual temperament (all three are skittish as all slings are) or color (although my G. Pulchripes is slowly starting to show color).
It would have to be Grammostola, from a pet rock to moody bit... and then there is the G. iheringi. Wow, fast grower, gets 8", tackles food with ravenous determination. The genera has a wide spectrum of T's to choose from both in mood/temperament and size/color.
Never heard of that species before aaaaaaaand now it's on the list. Thanks a lot lol.
Tough question here. I'm going to have to roll with Brachypelma since I've owned more and I suppose that means I've found them more interesting over the course of my time in the hobby. Buckley v. Valeo I suppose.
This is a poor question IMO. Better for what??? Define better, I can provide you a species from the Grammo that is a far faster grower and eater than any Brachy, is that better, NOPE, it's just different.... And in the end, it's often subjective.