Flaming hands
can cause you to ignite, though, via setting other things on fire. Damage wise, though, it's pretty bloody solid.
'Best mutation', though, is pretty subjective. For me, multiple legs or heightened speed have been my MVP mutations, followed by freezing hands. (Stun.
Lock.) Quills are g'damn vicious, and wings can make certain tasks (anything aboveground) basically a ridiculous cakewalk. (Hello saltwurm. What? You can hit me 'cause I'z flyinz? Ohohohohoho-insertprojectiledakka) Folks have talked about using fugue (time clones~) to make little artillery brigades (Equip self with grenades and/or missile launchers and/or flamethrowers and/or various!projectile-mutations).
I actually don't really like light mutation. Glowspheres are pretty easy to find (multiple arms means you don't have the 'i-needz-free-hand' issue) and floating glowspheres like to show up in the non-joppa merchants fairly regularly, so the ambient light doesn't really do much for you outside of a bit in the beginning, and laze just doesn't hit very hard. It pierces, sure, but so does flaming (EVERYTHINGISONFIRE) and freezing (stunlock <3) hands. Light manipulation is alright, but certainly not something I aim for when rolling.
Regeneration is awesome (hurrhurrkitekitekite). Carapace can be awesome, especially if paired with one of the thermal mutations. Horns are pretty kickin'. Most of the mental abilities can roflpwn. The gas effects are wicked useful in certain situations (P.S. Security doors. You are tough. The walls beside you, not as much. Also, salthopper? You are asleep. I am two screens away.).
I could keep going. Honestly, pretty much all the fully functional mutations are just great. Makes the
*band and Crawl most mutation systems I've seen (in games) look like sick little puppies