Season 7: IMO the best, cuz they have such a beautiful closeness.
Season 5: Spike's realization of love, their buildup of trust.
Season 4: I loved their banter, & tempting interactions.
Season 6: Such a sizzling, angsty season for them.
Seasons 2 & 3: Loving their earlier interactions. Reluctant allies.

i love them all for so many different reasons but with seven they were something amazing about it he became completely apart of the gang she trusted him above anyone else and all of the above