Hmm let's see... These are my favorite couples I can think of so far... I think they're all sweet in their own strange ways ^_^
-Kagome and InuYasha (InuYasha)
-Saya and Hagi (Blood+)
-Sango and Miroku (InuYasha)
-Keitaro and Naru (Love Hina)
-Tsubaki and Urabe (Mysterious Girlfriend X - yes I know the anime barely started, but still...)
-Katsumi and Roy (Silent Mobius)
yano was pretty messed up i felt really bad for nana
i think one of the sweetest is the special A couple kei and Hikari, cuz Kei obviously loves her despite being RIVALZONED all these years xD
i dunno about other genre though, the only one that comes to mind is arakawa under the bridge, but that couple has a long way to go from what i've seen so far cuz nino's really hard to read.
one may tolerate a world of demons for the sake of an angel.
Rin and Asa - Shuffle
Nagisa and Tomoya - Clannad
Chibi Moon and Helios - Sailor Moon
Bulma and Vegeta - DBZ [ I just freakin love the argumenitive (not a word, apparently!) couples ]
Haruhi and Hikaru - Ouran Highschool Host Club
Chi and Hideki - Chobits
Sakura and Li - Card Captor Sakura
Yukito and Touya - Card Captor Sakura [ don't hate ] Kuragone and Fai! - Tsubasa Reservoir Chronicles
Misaki and Usui - Maid Sama
Misaki and Usagi - Jounjou Romantica [ I love gay relationships too, apparently ]