My best guess is axe as being the most reasonable one, not only on attack settings, but also as being easy to own even if you got a new character on a new world:
Now commenting on the level range that I proposed:
Obsidian Lance: http://tibia.wikia.com/wiki/Obsidian_Lance
Excellent weapon for a level 20, very common to see around with a relative low price.
Naginata: http://tibia.wikia.com/wiki/Naginata
This one is quite rare and you won't get one easily without a higher level character or without a friend, but it is an extreme powerful choice to own on a level 25 knight. The baddest thing about it is the fact that it might have a high price always, since most of the people do not care about having this weapon and sell only to collector's/house owners. You can try also a http://tibia.wikia.com/wiki/Daramian_Waraxe if a Naginata is out of question, but the latter option is also quite rare to see people selling it or using it. It is a level range with "underground" utilization of two-handed axes, I don't know why.
Glorious Axe: http://tibia.wikia.com/wiki/Glorious_Axe
Not used very often, but still, a great option to consider (I personally appreciate the visual of it). The monsters that drop it are diverse nowadays, and the price for it is not that high.
Honorable mentions: Double Axe http://tibia.wikia.com/wiki/Double_Axe and Halberd http://tibia.wikia.com/wiki/Halberd
These need ones need to be mentioned. They would rather be a choice for people lacking money or just when there is a new server and you are a knight that want to kill mobs faster in order to get profit and experience. Later on these weapons will be an ordinary loot bag item for players, but it can be a quick and cheap option to deal a somewhat higher damage early on as a Knight.