If I could take just one thing it would be T3; I think that has the biggest impact for me as far as giving my metabolism a boost without the distracting sides of ECA, Clen, etc. I've run it with and without Clen, by itself it works ok, and Clen makes me hungry, which can be counterproductive when trying to cut...
If you have experience with clen, then you'll know what's going on and can stack it with T3, if you're taking 2 new things at once harder to tell what's doing what. Make sure to split your clen dosage as needed, some say to split T3, but I've always taken it (100mcg pill) at one time with no issue/sides other than the metabolism boost. Also, some people are super sensitive to clen or t3, so start really low until you know how it affects you. I had run a couple of clen 2 week cycles before adding in T3 so I could tell what was what, I know the combo is generally advised, but personally I'd rather not throw too many new things into the mix all at once.