For beginners its better to use cyke, because it beats the majority of the assists in the game. Cyke eats up tron, psy, capcom (not sure, maybe even), doom, cable, sent, and cammy.
Psylocke is only good for doom and tron. it gets owned pretty bad by cyke and capcom.
Cyke’s assist can be ROMed off of, and is great because it has good range and fires an optic bullet at the end. Cyke is also a great fighting character for a 3rd person. Check out the cyke threads for more info
Psylocke’s assist on the other hand stuns the opponent in the air for longer. You can also do a c.lk, launch and the opponent cant roll out of it if you time it right. Cable’s AHVB and ROM infinite works well with this assist. It is perfect with storm and mag (team MSP).
Doom is a pretty good assist at a near distance that will beat almost anything while started. You just need to time it right. Just have patience and time it after the assist. Doom’s assist goes well with strider, mag, storm, and sent for some corner rapage.
Capcom is a good assist but needs to watch out for doom and cammy and cyke. Capcom best works on psylocke and sentinel & storm. You usually use sentinel/storm/cable with this assist.
Cammy is one of the best characters to be left with. She has a dash just as fast as mag’s (foward dash, not back 
). Her assist goes slightly diagonal and can own anything if timed right. She is used with mag, cable, and storm.
Jin, which is not used very much is a VERY powerful character as far as i see. I try it and it beats everything i know. I still dont know why they dont use it in tourneys. Weird, but IMO jin is good against mag, storm(runaways will be hard) and sent.
Tron is a very useful assist. With mag and storm, it can do some rapage. This is one of the most damaging assist when connected. Oh yeah, it also works well with sent. This assists owns capcom, and if timed right a few other assists.
In general, there is no dominant assist. If youare the underdog in the match, try to time it right. You can get a hit if you hit your assist slightly before or sometimes after. Be patient, and watch out while against cabl storm and sentinel. Here’s some more advice
Playing against cable: Limit your assists. If he has lots of bars, make sure you are close when or after you assist. Usually teleport/sj/ or tri jump near.
Playing against storm: Her hail moves will ice ur assist if not careful. She will most likely have lots of bars due to tri jumping and runaways. A sentinel beam or typhoon from storm will stop her. AHVB’s will work too. If you are currently with mag, just stay close. With strider keep teleporting her direction.
Playing against sent: Ok as i learned from one guy, you can escape rap by sent’s HSF. You pushblock on the laser or rocket punch, and then jump AFTER the first set of drones. With cable, you should AHVB afterwards, storm & mag sj then dash foward, sent counter with a HSF of his own.
Playing against mag:He has nothing that will seriously injure your assist. Watch out for sent DHC’s or Cable’s AA counter into a AHVB.
). Her assist goes slightly diagonal and can own anything if timed right. She is used with mag, cable, and storm.