my oldest char is also still one of my favorite; is basically a chimera, genetic chaos isnt as bad as you’d explect, for the most part it tends to bounce around between slight variations (i think the way mutations have been coded) so you may end up with a thick tail, club tail or stinger, it usually doesnt stray to far from that.
the other thing is Hyper-metabolism; this beauty basically converts any excess nutrition into hp, so any fight is not only going to fill you up, you can actively heal even severe damage in a few seconds (provided you have been butchering those zombies)
basically you lend yourself very well to fighting by the end of the tree and the resulting mess basically sustains you.
it does bring up an interesting point; is there going to be any revision on the less interesting mutation trees?
troggs, a while back i stumbled upon this thread on reddit, never even considered troggs till then:
basically the best pre threshold mutations and is a good secondary once you have made post on the one you want.
as for rat, while i cant talk about post threshold mutations i can honestly say its a really good beginner tree pre, it shares a lot of the good trog mutations; NV, substance tolerance, poison resist, saprovore disease immune; basically a bunch of really useful traits for a starting character.
negative ones such as troglobite and solar sensitivity are good re-enforcers for good practice at the start of the game; ie only going out at night.
rapid meta; you need to eat more but it also heals you faster
others such as near sighted can be countered by a pair of glasses, or are minor annoyances at best
imho the only big dent in this is carnivore; which sucks 
on the two characters i have with these mutations the only one on both that prohibits normal gear is the paws mutation.
the best part of this is most if not all of these are easily attainable right at the start, just find a cave with a rat king and hang out with him for a while, the rat king never really seems to want to attack you, food isnt hard to come by while waiting in here; munching on the meat supplied by the rat population you murdered to get into the cave.