The best PvE ship is the one that you enjoy flying and fighting with. Nobody else can tell you what you enjoy.
If you have access to Cytoscramblers then I'd go Chieftain every time. It's like getting Large-equivalent firepower from your Small hardpoints. Two efficient ones is 60+ anti-shield DPS all by themselves, so you can go heavier on anti-hull or anti-module elsewhere.
Krait MKII if you want something that can do anything. It's a bit sluggish off-boost, but the firepower is great.
Krait Phantom if you want to outrun every other medium ship except Mamba. Still good firepower and can make use of power-hungry weapons. Nimble in the blue zone, especially under boost. Decent shield tank when using Guardian reinforcements, decent hull tank if you use lots of HRP-s.
Chieftain is one of the nimblest mediums with no need for boosting. Decent jumprange of around 30 ly with the v1 FSD. Very versatile hardpoint compliment with many, many viable loadouts. Decent internals for a combat ship: you can have class 6 shield, class 5 cargo rack and another class 4 slot for limpets, SRV-s, fuel scoop or something else.
Agree with the Krait sluggishness. Kraits strike me as the squishy Wizards of the galaxy. Vast DPS and a familiar (SLF) that does damage for you, but your defenses are never as strong as they seem, and even the Phantom seems pokey unless you're staunchly in the blue. You see this in installation defense scenarios. They hurt bad if they hit you, but once their shields are down they crumple like paper. Plus, the offset large hardpoints are set far enough back that gimbaled lasers can be blocked by your own hull if your target is below your eyeline, and that's always bothered me.
Chieftains are the Rogues. If they're behind you, and most of the time they will be, you're dead. If they're in front of you, then something has gone very wrong and they're dead.