If Sasami were older, she would easily be the best one, because she's really one of the few nice people on the show. But she's not, so I say Ryoko.
As for Tenchi in Tokyo... [spoiler]Sakuya's out because she's Yugi's shadow,[/spoiler] and Yugi's out because she's also too young... so I still say Ryoko.
...You know, there should be more kids in Tenchi. Sasami and Yugi are left out of most of the love triangles because of the lack of kids.