Most of Sektor's available strings are practical, except b3u4 which you'd want to avoid altogether.
114 is his fastest launcher and also very damaging. It's also safe on block, probably +2. But it's high and doesn't advance that much.
21 is fairly quick and gives u lots of options. 212 missile is a good block string and has pushback. 213 hits overhead on the last hit. since 21 is positive on block you can also throw afterwards. Even 21 up missile could be an option but it's playing with fire.
f13 is fairly quick and forward advancing. The second hit is low so you can throw it out there once in a while. If you have meter you can actually commit to a f13 tele because if it's blocked u can get out with ex-air dash.
f43 is very forward advancing but a little slow on startup. The second hit can be neutral crouched so there is mind game here. Use this string sparingly.
f212 bb is perhaps Sektor's best string. There's huge advantage on block and you can do just about anything: straight missile, up missile, run up and throw...If it hits you just tele and grab around 40% with 1 bar.
b3d4u3 is great too and very hard to block. It's first 2 hits are low and the other two are overhead. It switches sides so its great for putting someone back in the corner.
b3/b1 into ex straight missile is great because it launches and its completely safe. Or you could just use b3/b1 into tele if you have meter and get out with ex air dash.
b2 has great range and can be cancelled into anything.