definately oz. i know willow was older when she met tara, but her and oz's love was more powerful. they were perfect for each other; they balanced each other out. most importantly, their relationship sprung from friendship. they were comfortable with each other and it wasnt all about passion, like willow and tara's relationship.
and so what if they cheated on each other? they went through more trials but they survived through them. oz forgave willow for the xander fluke and willow forgave him for the veruca thing. never once did they leave each other for good like tara left willow - and that was mostly because she was scared. willow and oz's love was never forced - did you ever see willow hypnotizing oz into forgetting an argument?
plus, we all saw what happened to willow when oz left. the only reason she went dark willow after tara's death was because her magic obsession was already rising to an explosion point. had oz died i doubt even xander would've been able to stop willow.
so all in all, i say willoz was the best relationship. and i cling to the dream that they'll reunite sometime when willow's travelling the world - but that's just me.