Sipahi of the Porte ?
In the medieval campaign, Turks are indeed your best bet. Hungarian could also tempt you (mounted X-bows, HA, Szekely, jobbagy) but you would then have to deal with the Pope and the marrying of your princesses (on the other hand the heavy infantry available to all catholic factions is yours to use while Turks only have true heavy infantry as from the Late period with quite heavy building requirements)... Spain or Aragon could suit yuou if you want "mounted-javelinthrowers".
In VI, my guess would be that the Welsh are most likely to catch your fancy (but they do not have any heavy infantry at all as far as I can recall)
Regarding archers in siege they are pretty useless in MTW once the fortication is a keep or better. If they defend they can only fire with some accuracy once the assaulter is inside the keep and the assaulter has a unexplained tendency to immediately engage your archers as soon as it has breached the gate/wall ...
They can bea bit more useful on the offence, not that they can fire more efficiently before the gate/wall has been breached but they can deal with "bottlenecks" created by high valour units plugging holes.They get killed by arrow towers and the like in the process but sometimes do more damage than H-t-h units that can be slaughtered with inlficting anything other than sympbolic losses. For instance, if you do no have any siege equipement, can only breach the gate and cannot overcome an unit that is just too good for your HTH units (a jedi byz prince comes to mind), peppering it with arrows can help you a lot. Arbalester and X-bows are better at that kind of job though. If the fortifaction is just a fort archers can be quite useful when assaultingh since they can destroy wall and gates by shooting at them (I guess you knew that but well ...)
Hope it helps,
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