In my opinion, Tenderfoot is not the simplest card in the game, based on its being a unit.
I would assume that, most likely, a spell or upgrade would win that title.
Neither units, spells nor upgrades are simple in general, but between them, you generally need to ask more questions about a unit than you do for a spell or upgrade.
i.e. You usually aren't concerned with as many things when considering the effects of a spell or upgrade vs that of a unit. The 'list of things to check' is drastically smaller for Rich Earth vs Tenderfoot for instance.
For a unit you need to ask questions like whether it has sparkshot, is patrolling, is exhausted, is in jail, whether its a copy of something else, are its stats being different than printed for some reason(s)? That 'list' is much larger than the 'list of checks' for an upgrade, for instance.