For Valentine assists, this is all I can offer:
Cr.mk (my choice), is used do to it being a long reaching low. It has good blockstun and hitstun, keeps the opponent grounded on hit or block, can be used to link chains that otherwise would not, can be used in throw mixups as a throw whiff guard (protect the point after a throw whiff), can be used to confirm off of heavy pokes, etc.
Throw is very good for beginning players, as its crumple allows easy followups. Keeps the opponent honest, can control the opponents movement just by calling it (They will probably jump it, or rarely try to tech it). Downsides are that it scales its followup, and its a throw assist, not much utility.
S.hp is very popular as well. Its hitbox is great, it can avoid lows, act as a long range poke, has good blockstun which combined with its range allows for quick run ins after its use. Can be hard to confirm off of max range, but it can't have everything.
Hk Bypass is very much slept on and considered a gimmicky assist for no good reason. Its Pros are that it deals 1275 in 1 hit, pops up on hit for a followup, allows heavy links with almost all characters getting better damage out of assist combos, can be used to confirm off of certain supers and commands, such as dynamo and diamond drop, has great blockstun that can keep unsafe moves safe, keeps the opponent honest at midrange, and forces attention at mid to long range. Cons are that it has bad recovery, very small vertical range (Which is a big deal for people). It is a great assist, and should see more use
Honorable mention for Mortuary Drop (Similar utility to throw, however untechable, has higher damage, and is fun to use imo) and s.mk (Slightly usable as lockdown due to new hitbox).