As a mono-white creature, Gandalf the White can easily fit into any deck with white mana, making it even more flexible
I would strongly consider bolstering that ability count to two in the list with either Temur Battlerage or Double Cleave
Time for top ten smashing
Whilst the first clause that triggers when being attacked may come up less often, the two spells a turn one should give you a steady stream of cards
While a four-mana 8/8 reach with evasion isn't exactly broken in Modern, and it is annoying that its backside's mana value is two, which means it dies to things like Fatal Push, March of Otherworldly Light, and Prismatic Ending, the opponent will lose the game super quickly if they don't have an immediate answer
The middle of these rankings is a perfect spot for the upsides and downsides of this commander