And so we end up at the rares. You can only have 4, so which ones fit best in the deck? (Spoiler: Not rare dual-lands because they aren't worth the slot)
The first mythic rare I chose, is the whole reason I wanted to make a burn deck: Nahiri's Wrath. This is by far one of my favourite cards and is in itself a win-condition in my deck. Because my hand size almost always stays really high, it generally deals over 10 damage, without even madness-ing any additional burn spells, to just obliterate my opponent for the mere cost of . It suffices to say that if I draw this card, my average win time is 5 turns.
Then comes Cut which is good as a cheap removal. Its flip Ribbons is good as an end game finisher and it doesn't matter that much if I discard it, since I can still use the aftermath. Pretty simple.
Its brother is Insult / Injury. Now this can enable me to do a whole lot of damage in a turn. The only downside is that it's a semi-late-game card, and I'm trying to make my deck as fast as possible. The aftermath is still always useful as both removal and burn and can be discarded.
Finally there is Fateful Showdown. This is another card that works well with my large hand size. In addition, it enables some madness-effects and it lets me refresh my hand if I run out of options or burn spells.
Two other rares I've considered are Burn from Within and Harness the Storm but I've scrapped them mainly because they're too 'late'. However, I would love to hear your opinion on them, and/or other rares.