Mahjong Stars

Pure efficiency: 2S, in all situations, the edge tiles (1-9, 2-8) are having significant disadvantage over the middle tiles (3-7) as their improvement tiles are numbered significantly less than their middle counterpart. Here, the 446M can be improved with [2M, 3M, 4M, 5M, 7M, 8M] (24 total) while 224S can be improved with [1S, 2S, 3S, 5S, 6S] (20 total), and which is better now better seen.

Pure Efficiency: 9S, as the only improvement to that is 8S or 7S (8 tiles total), which is considerably worse than 4S pair's improvement (include 2S, 3S, 5S and 6S. 24 tiles total).

For Pure efficiency: Go with 2S/8S, as discard any of them still allow this hand to have 48-52 possible tiles to improve (since singular tile is always worse on speed than a waiting shape) For Maxium worth hand: Go with 2P, 4P or 9P, as if someone may desire a 3 color straight of 678 and all-simples.

Consider situation, the "West" and "North" will be the first to leave the hand as 23456 (S) creates 1-4-7 wait and 78 (P) create 6-9 wait. Since there is 3 Riichi stick already on the table, and 1 Dora is visible in hand, the ideal direction would be Riichi+Pinfu+Dora 1 (2600+3000). But here is how I would consider: If one player already declared ready (anyone), I will consider cut "West" first. If no one is in Riichi, and 1 "North" is visible, I will discard "North" then.

Depend on the actual stage... Since the table matter is not given, I will assume all the waiting tiles are available. In this case, I will discard 3P/4P since 3344 have duplicated waits (well, if looking for 1 pair of identical sequences, 6M/8M is the choice)