Alright, so let's work with Cell Saga Krillin. Watching his fights, I'd be inclined to put it down something like this:
Round 1: Krillin. Base Genos has far less speed, power, and combat training (and probably experience) than Krillin does, while also appearing to be much less durable. Also, he can't fly, and he's more reckless by far. Not a good combination. Krillin regularly takes the kind of hits that base Genos can dish out with his machinegun blow, and it's unlikely that the incineration cannon will even manage to hit him, let alone significantly damage him, at that level of speed and power.
The only hope Genos really has, in fact, is his ability to blow up the space of a city block centered on himself, as he did to counter the mosquito swarm in Ep 2. Given the (lack of) damage apparently caused to surrounding architecture by this move, though, it's unlikely that it'd much harm somebody who frequently survives getting thrown through boulders.
...actually, there's that, and also the same kind of blast he used to destroy the House of Evolution HQ. DBZ characters often just inexplicably stand in front of their opponents and watch while some large, excessively destructive beam weapon is being prepared at them, so it's possible that he'd just try to tank it.
Round 2: I'd actually be inclined to go with Arms Mode Genos. His speed's at least pretty on par with Krillin's, he has some flight capability, and he can regularly spam out more destructive power (e.g., tearing up cliff walls as in the Saitama fight), than, to the best of my recollection, I've ever seen Krillin successfully tank. His durability has been greatly enhanced as well, to the point where Krillin may actually need a Kienzan just to hurt him in the first place. Given Genos's record at this point, though, it's not unlikely that he'd get careless enough to actually let that Kienzan connect, which would be a game-breaker.
Genos being the cyborg that he is, Krillin's other abilities - namely, sensing/hiding power level and his Solar Flare, - are just less effective answers to Genos's Lightning Eye and his ability to scan for life signs.
Round 3: Combat data on Genos's G4 upgrades is limited at this point, but so far he's demonstrated comparable power to Arms Mode, while also being capable of moving faster than the speed of sound (in atmo) with negligible acceleration time. Since this means that even the slightest, most last-second effort on Genos's part allows him to easily dodge the Kienzan, (and anything else Krillin's got, really), I'd go with a pretty solid win for Genos here.
