Steve the question is what do your RG decks need help against? I think you got the main players in there already, so why not use the last slot to address a weakness? I could see several Gruul cards as viable options depending on what you want to shore up.
Most likely Baneslayer, it's the most 'plain' in what it does as all the others contribute to archetypes while being strong on their own, and the closest one to Baneslayer is Avacyn which isn't the same but the differences are exciting enough for me to warrant inclusion over her.
That being said, if you chose to cut one of the others it def wouldn't be the end of the world as Baneslayer is really strong. There are a ton of white 5s that are good enough.
Baneslayer is the only one of those I do not and would not run. I can justify running Baneslayer despite it's weakness to removal because of it's strong impact combat and the race. However, the other stellar white 5s bring similar power to the table, either through synergy or sheer board presence. And those cards laugh in the face of removal.
Baneslayer maybe be slightly weak to removal and doesn't fit into any one clear archetype, but She's such a house. Those stats are too good to cut simply because she "dies to doom blade", and she may not have a clear archetype but there aren't too many decks that are upset with playing baneslayer.
I'd cut ranger because between Hero of bladehold, and Angel you have plenty "army in a can creatures" and I think ranger is out classed these days.
I still like ranger a lot and I think it's the best of the army-in-a-can variants. It makes the most amount of tokens which is important with anthems which I think most cubes have a decent amount of between Spear/Glorious Anthem/Honor/Accorder Paladin/Gideon/Elspeth ultimate/etc, and the flying clause is pretty important to for forcing in damage. You not only tie between all those for having the most P/T in a vacuum, but you also tie for the most amount of damage through the air, which can't be ignored.
I still like ranger a lot and I think it's the best of the army-in-a-can variants, and maybe the best 5 drop. It makes the most amount of tokens which is important with anthems which I think most cubes have a decent amount of between Spear/Glorious Anthem/Honor/Accorder Paladin/Gideon/Angel of Invention(maybe the 2nd best 5?)/Elspeth ultimate/etc, and the flying clause is pretty important to for forcing in damage. You not only tie between all those for having the most P/T in a vacuum, but you also tie for the most amount of damage through the air, which can't be ignored.
If Baneslayer was uncuttable/deemed necessary, I would cut Avacyn next, but I could also see cutting Reveillark. Personally I would never, but I get that other playgroups may not have seen it play as good due to the demands it asks for success so it's understandable even if I wouldn't agree with it.
Do you support Daretti/Goblin Welder/Academy Ruins? If so, maybe Mind Slaver since it's more of a 'fun' card and does something different (well, one person is having fun lol), but having a colorless piece of removal is nice in those shells too, along with being something a heavy green deck would play.
Scuttling Doom Engine is also an underrated option, if you want a finisher instead of a combo piece/removal.