Terrestrial = Gorilla. Marco had the right idea with this morph, you have excellent senses, can use whatever weapons/equipment are dropped, you're stronger than most bipedal species and you can climb/jump quite effectively. As a distant relative to humans it wouldn't be too dissimilar to the normal human form so it would be easy to control and get used to.
Animorphs = Howler. Obvious reasons ^^ Vast combat experience makes this a very ideal morph as well as it's howl, strength, speed and senses.
General sci-fi (excl reality affecting level) = Species 8472. Almost unstoppable due to it's evolutionarily peaked biology. Fast cell regeneration, survivability in all environments (incl the vacuum of space). No biological weaponry or poison will work against it. Pretty much the only thing it's lacking is acidic blood
Whilst they don't generally use weapons there's no reason why they can't since their technology is more advanced than humans. Oh and telepathy ALWAYS handy!