Gordon
It was Country Spirit that placed Gordon into my top 5 favorite costars with Charmaine. They both did a wonderful job in this series and I didn't mind that their love story was dragging. I enjoyed many moments they shared in Country Spirit, especially their genuine friendship. When they later became a couple, it got boring and I didn't like them as much as before. Regardless, I think they worked well with one another. CS was actually my second series of Gordon & Charmaine. I first saw them in Crimson Sabre 2000. Their chemistry was just so-so, nothing special; therefore, I won't put anything down for this series. To me, Country Spirit alone was more than enough. The chance of them collaborate again is zero to none so I'm really glad this couple left me a series to remember; namely Country Spirit.

Stephen
I started liking them from Angels of Mission. Stephen wasn't the lead in this series but he played his character well and his chemistry with Charmaine was great. I really enjoyed watching their scenes and both of  them were really cool. Then came Strike At Heart. I'm not a fan of wuxia series but I was excited in seeing Stephen & Charmaine as an official leading onscreen couple. They didn't disappoint me. Both were great, as always, in their roles and while watching this series, I kept hoping for Charmaine to fall in love with him. Their characters in SAH were pitiful. Anyway, I feel that this couple share an undescribable chemistry. I can't quite tell what it is but they just have this great onscreen presence. It's really sad that TVB stops promoting Stephen after Strike At Heart. I really, and I mean REALLY want them to collaborate again in the future.
Steven
My first serie of them was Return of the Cuckoo, I guess they were ok in there but I didn't care much for them in that series. All my engagement and attention were of course on Julian & Charmaine. I have always perceived Steven's character as an "outsider" in ROTC. The moment he made his apperance into the peaceful town, everything started to take dramatic changes. I didn't like the fact that he came into JC (Julian & Charmaine)'s relationship...Then came Perish in the Name of Love. "Wow", that was all I can say. I was addicted to PITNOL. 'Till this point, I can't even distinguish whether it was their tragic love story or their actual chemistry that made me toally fall for them. All I know is that I was in love with this couple after PITNOL, enough to put Steven into my top 3 favorite co-stars with Charmaine [He also happens to be one of my favorite actor]...I really do hope TVB would pair them up again,....preferably in a modern series. I want to see if they'd be able to spark the chemistry they once had in PITNOL.
Roger
With Roger and Charmaine, it was love at first sight. The moment I saw the two of them bickering in A Herbalish Affair, I knew I was going to like this couple. When the series AHA came out at my place, I was extremely happy [On a side note, I started liking Roger from "A Threshold of an Era" so he has always been a favorite of mine] So putting two of my favorite actors in a series, and even further more as a couple was a dream come true to me [not always though]. The first 10 episodes or so of AHA are definitely a must watch. The chemistry they shared was wonderful. Then came Glittering Days, I was overjoyed. Roger & Charmaine managed to live up to my expectation which was incredible since I had high expectation for them. Their performances in GD were good and once again, they pulled off undeniable chemistry.
Julian
There aren't any words to describe how much I love this couple. They have amazing chemistry on & offscreen. I absolutely adore their pairing in Return of the Cuckoo. As for Point of No Return, I can't bring myself to watch it the second time around mainly due to the pains they both suffered in middle of the series, it was heartwrenching. Anyway, I recall that Julian was the first guy I ever like with Charmaine. I watched ROTC quite long ago but their chemistry left a deep impression in me. Their love story in ROTC is very memorable. JC forever remains in my heart as my most favorite onscreen couple. I am just really in love with them. In fact, I managed to read most of their fanfics at winglin. I really want TVB to pair them up as couple in series again. The chance is pretty slim though; but I'm still hoping....It is quite possilbe for them to collaborate again, not necessary TVB but maybe Mainland, China productions.
#4 - Stephen Au
#5 - Gordon Lam
#3 - Steven Ma
#2 - Roger Kwok
#1 - Julian Cheung
Julian & Charmaine
#6 - Sunny Chan
Sunny
I was undecided between Sunny & Gordon for my #5 favorite co-star with Charmaine. At the end, I chose Gordon because I am very much in love with him & Charmaine in Country Spirit. With this said, Sunny has always been my #6 favorite onscreen partner with her. I actutally first saw this couple in "The Flying Fox of the Snowy Mountain 1999", I had no interest in TVB back then so I had little memories of this series along with the pairing [On a side note, it was also my first time seeing Charmaine onscreen] Anyway, a few years back I decided to watch some of Charmaine's old series and "Detective Investigation Files IV" was among them. Sunny & Charmaine made a nice couple and their chemistry was quite good. I became a fan while watching this series. After finishing "DIFIV", I wasn't hesitate at all to rent "FFOTM 99" since I didn't even have a chance to watch the whole series back then anywayz. I thought this series was enjoyable despite all the complaints others made about it [how TVB completely changed many important parts and characters, etc...also note that I had no prior knowledge on the novel] To sum it all up, I like the chemistry this couple has and I believe Charmaine once mentioned that Sunny (also Gordon & Gallen?) has taught her a lot during her earlier years in the entertainment industry [?] The chance of them collaborate again is quite promising now that Sunny's back in TVB -p
#7 - ?