2k1 is my fave KoF, it's technical excellence hasn't been surpassed so far. 2k2 went back to oldstyle and came up with BC cancels which are very nice and flashy and all that, they helped to make it the #1 choice for pro players but I just love the little intricacies of 2k1 and the revised striker system. It's no striker bombing á la 2k anymore, the ratio system with the option to choose up to 3 of them is combo heaven for me, and just look at the vast selection of striker charas, it's almost a KoF/SNK best-of.
Graphically and sound-wise, 2k1 falls off quite a bit from the original SNK predecessors. I remember the horror when I first saw the chara select artwork, all the charas looked like genetically mutated smurfs. Thankfully, the old sprites based on Shinkiro's work are still there. Backs are a quite mixed bag, some of them are kinda nice but can't hold a candle to the older KoFs, specially 95, 98 and 99.
I should add that 2k1 wasn't love on first sight for me, took me a while to appreciate it. When 2k1 came out, fans still moaned SNK's demise and so did I, that's why I almost had an allergic reaction to 2k1 at first. It wasn't before I sat down and really spent some quality time with it that I found out about it's finer points and fell in love with it.
Btw., there still are lotsa interesting KoF2k1 threads in Gaming, including some striker combo threads with really great stuff, check it out if you haven't already and get inspired!