Which season is most worth watching (4-5)?

[Guilmon and a shotgun]

Watch the first 25 episodes of Season 4 and the last 25 of Season 5 if you want the best of both worlds.

Haha, nice remark Wise i'm noting that down

But more to the point, why not watch them both? I will give you fair warning though, if you wanna not be dissapointed then watch 5 first. Season 4 was the one where they decided to change a crucial element of Digimon and as a result not many people like it for that.