Croatian versions of those pages don't mention Ratimir as ban, but as knez. So does his namepage on English wiki.http://en.wikipedia.org/wiki/Ratimir_(Pannonian)
http://en.wikipedia.org/wiki/Ban_(title)
You might be right though, might fit better with a duke title(only thats a Knez(which in Serbia is a count):wacko(was doing reserach for Serbian, Bosnian and Bulgarian titles at the same time so I might have gotten confused with Croatia)
I can understand the confusion, since the a) various titles were used which we need to abstract into 2 tiers b) those titles didn't have a "fixed value" c) they could mean one thing at one place and another in a different one.
IMHO the lowest common denominator is župan, which could account for the baron-level title for both Croats and Serbs (though there were great župans in Serbia which equaled duke/prince titles).
Next up the list is knez title. It designated a ruler over a nation (knez Srba/dux Serborum or knez Hrvata/dux Croatorum) at first, but when Croats and Serbs established their kingdoms, knez title was lowered in the perceived realm hierarchy - most often a ruler of an area that would account for a single province on the game map, and with the title translating into "comes" - count in Latin (it's been used in Bosnia in this sense too). After the dissolution Empire of Serbia, knez title once again rose up the ladder, though only for a short period of time (1/4 of a century), to be replaced by despot.
As for ban, I stand by the argument that it's should be a duke level title in-game, represented in those areas with significant Hungarian influence - Croatia, Bosnia, north-western Serbia. First know ban of Bosnia (Borić) was installed as an Hungarian vassal. There was also a brief period of banate of Zahumlje/Zachlumia, when the future Andrew II of Hungary established some sort of control over this area. Same goes for north-eastern Serbia, there was a short-lived banate of Braničevo (accounts for a part of Vidin province on the game map) when the area was under Hungarian control.
All that said, I'm still unsure as to what could be used for "duke" in Serbia, but veliki (great) knez seems like a best fit. There were, if not at the same time a great knez of Hum, Duklja, and the two 14th century knezs of Rascia were sometimes referred to as "great knez".