For the record, I like S1-4 Willow way more than S5-7 Willow but S5 Willow is still totes my favorite character and I'm even inclined to try to defend S6-7 Willow because of how much she gets bashed by fans, the creators and the other characters. However, I'll stick to mid-series Willow for the moment because I sense unfair treatment.
Willow S1-3? Genuine contender for my favourite character. Willow S4-7? Held in the same esteem I hold Spike.
I actually think S4 Willow is very much in the same mold as the S1-3 character. She grows and changes a heck of a lot through those seasons and the superficialities may be quite different in S4 (interest in a woman, short hair, no overalls or tights) but it's still a logical and attractive outgrowth of High School Willow. She's still trying to balance science/computers with magic. She's not all powerful and her spells take thought and concentration. She may have given up Ivy League dreams but she's pretty determined to have a successful college career, socially and academically. She's still one of, if not the most important, emotional supports for Buffy and the person that Buffy confides her fears, her romantic aspirations, etc.
And while I may see negative changes in S5, they're pretty much all negative structural changes. Like her role as Buffy's emotional support is supplanted by Dawnie-Come-Lately or her ability to go toe-to-toe with a goddess undermines the dramatic need to have the Scoobies be outgunned and overcome their challenges with a lot of efforts and some surprise. Also, she really should have been positioned as "bi". But Willow, herself? Still incredibly likable and I'd argue the most likable character in S5 and someone I'd love to have as a friend. Because she:
a) tries her best to be supportive of Buffy in all of Buffy's angst and iss-yews while asking very little from Buffy
b) offers Giles constant friendship from she archives his books/begs him to stay in Sunnydale/validates his presence at the beginning to stopping his bleeding and delegating Xander to get him to a hospital at the end of the season (BTW, Willow's horrified beyond horrified expression as she's desperately trying to stem Giles's bleeding in Spiral means more to me than Buffy's beatific expression as Giles tells her how always right Buffy is in the same ep)
c) Completely overhauls Tara's quality of life. Two years ago, Tara was being beaten down and abused. Now, Willow dotes on Tara constantly with compliments, provision of friends, big birthday and round-the-clock-care and a restoration of Tara's sanity when she was brainsucked which is more than Glory's other brainsucked victims got.
d) Offers Xander more friendship in this season than he does for her, in moments both big (put 'em up in The Body) and small ("I think you're cool!" in Fool for Love).
e) Defends Dawn on a number of occasions to Jealous!Short Tempered!Buffy and engages Dawn's attention with math tutoring, chess and magic.
f) Displays heroism in pretty much all of the fights that she gets involved in, including but not limited to saving Giles/Xander lives in Listening to Fear, fighting Glory through to the end and after a spectacular kicking of her ass, coming back to give Tara her sanity back, fighting the invisible demons in Family and keeping her composure to put up a wall in Weight of the World through the flaming arrows and to re-trap Olaf. (The only exception is that I think Willow should have argued with Anya when Anya offered her life for Xander's in Triangle and insisted that she should die for Xander instead. It would have been a) in character and b) very funny to see Anya and Willow, then, start arguing about who should die for Xander.)
g) Calls on all skills that she's personally developed (without a Watcher or a sacred duty or formal education or the camera on her...) including the old mainstays of computer hacking and magic but also risks her health to practice teleportation when it was effective against Glory and fixes the Buffybot.
So S1-3 Willow may be the closest to my heart but S4-5 Willow is still totally awesome and I'll defend her to death. S6-7 Willow is a little more dicey. Maybe I'll come back and discuss that later....