Alara Reborn is 100% multicolor. All 145 cards are gold. This is obviously the highest percentage. But how highest?
The first record-holder was Limited Edition: Alpha, with 0% of its cards being multicolored. This record would be tied by every subsequent set until Legends came out with the first gold cards. Of Legends's 310 cards, 55 were multicolored, meaning that 17.7% was now the mark to beat.
The next heavily multicolored set was Invasion, which may or may not have set a new record. For the purposes of this exercise, we're not considering Split Cards to be "gold", so Invasion's 61 out of 350 only gives it 17.4%. Planeshift's 35 out of 143 gave it 24%, which took the lead until the next set, when Apocalypse had 38/143, or 26.6%.
Jumping ahead to Ravnica's 306 cards, we have to decide what to do about Hybrid cards. Again we're going to not count them, because we're more interested in pure "Gold Cards", so there were 16.7%. Guildpact (41/165: 24.8%) failed to clear the bar, but Dissension had 49 out of 180 (counting Split Cards this time, because the cards were Gold at the same time) for a new record of 27.2%.
Since we're not counting Hybrid, we'll skip Shadowmoor and Eventide (although if we did count Hybrid, they'd both set new records at 36.5% and 47.2%) and go straight to Shards of Alara, which came in at a relatively sedate 47 gold cards out of 249 total: 18.9%. Conflux came up a tiny bit short at 36/145 (24.8%), which means that the grand champion (pre-Alara Reborn division) is Dissension!
|Shards of Alara||249||47||18.9%|