Raijinken
Smash Master
Firewall is the one you speak of. It's a tradeoff, and one I'm typically fine with making. It deploys so quickly that you can use it to relieve some of the inevitable rushdown pressure that Robin struggles tremendously with, and you can cover landings a bit better at the cost of combos and range.There's this one Arcfire that drops a pillar of flames at your feet that's actually really useful against recovering opponents because it's lightning fast and apparently has infinite downward range. Don't play Robin though, so IDK if it's actually good enough to give up the default.
Less that Robin's customs are bad and more that his specials were bad to begin with.
Given his recent defensive combo game, I'd probably rate default as best, but as you say, his specials are pretty bad over-all.
Worth noting, though, the fireball will not proc the column on contact with an enemy. Only works if it hits the ground (maybe a wall? Honestly never bothered testing that).