Spice and Wolf has been adapted twice from the same light novels, and neither adaptation continues the other. That makes the choice simple in one respect and awkward in another.
The simple part: start with the 2024 remake, Merchant Meets the Wise Wolf. It runs 25 episodes, covers the first four novels, and its second season is confirmed for 2027. Nothing before it is required viewing.
The awkward part is that the two versions did not cover identical ground. The 2008 season adapted the first two novels and the 2009 season adapted the third and the fifth, skipping the fourth entirely. The remake filled that gap, adapting the fourth novel for the first time, but it has not reached the fifth. So the older run is not simply a lesser version of the remake: its second season contains an arc that has not been animated again since.
That is why the 2008-2009 entries sit below the divider rather than being cut. They are optional for a first watch, and they retread material you will already have seen, but the second season is the one place the fifth novel exists on screen. If you finish the remake and do not want to wait until 2027, that is where to go — accepting that its first half covers a book the remake already handled, and that it skips the book the remake did best.
Within the older run, the single-episode OVA sits between the two seasons and was made as its season two opener, so watch it in that slot if you go down that path.