C&C95 - the chem warrior is kinda useless by virtue of its high tech level, even though they are kinda cool. I guess it's kind of a testamanent to C&C95's good balance that practically all of the mainstream units are good in certain situations. Even mammoths are useful in SP as damage sponges.
RA - Yak, MRJ, MGG; but basically anything other than the 3 main tanks and V2's is pretty useless.
Rangers? pointless when a 1tnk is £100 more. Shock troopers cost the same as a heavy and get massacred by artillery; so why would you ever build one?
Tesla tanks cost almost the same as 2 heavy tanks and 1 V2 will kill one. I swear I've seen one taken out in a single Artillery shot.
Mammoths too slow. MAD tank too situational outside of amusing yourself by detonating 3 against the AI.
Ren - the tiberium auto rifle (the Sydney character) was pretty useless. It shot slow moving tib clouds that did next to no damage against tanks; a basic rifleman could take one out.