As the Pokemon strategy forum moderator, of course Taillow ^_^
Altaria's role is usually replacable by someone else. If it's one with dragon dance, Salamence can use the same moveset and do it a lot better. If it's a special sweeper, then quite sure that many other things in the world, or even Dragonite, can use the same moveset again and do a better job with it. Its stats are very rounded out, but in the world of RS where there's EV, it's not so great...
Swellow on the other hand, is irreplacable. No one else can use the exact same moveset of Swellow better than Swellow itself.