Aside from being a human, which is a very relevant creature type in Commander, Modern and basically every other MTG format, Esper Sentinel is good because it reads: Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays X, where X is Esper Sentinels power. Its a bit like Rhystic Study, but the cost can be far higher if youve got things that make humans stronger or you can put +1/+1 counters onto Esper Sentinel
The Necrobloom is grindy, explosive, recursive, and absurdly rewarding to build around, which is why it earns the top spot on this list
Each creature you control gains an ability that puts +1/+1 counters on itself equal to the mana it produces whenever you tap it for mana
To saddle a mount, you need to tap any number of other creatures with total power N or greater, where N is the saddle value