"May I ask, since you both have done all of these, which would you say is your least favourite/scenic?"
The entire region is beautiful. I have been there 3 times in the last 4 years and what amazes me is the variety of landscapes between the various hikes. In pure beauty, I guess I would vote for the Mannlichen-Kleine Schleidegg trail, with one caveat. Go early in the morning. As beautiful that trail is, by 930AM, it becomes very popular with hikers and it takes away from the beauty. If you want to feel alone with your girlfriend/fiancee and are looking for the hike where you can privately propose, that would be various sections of the North Face trail or the trail from Murren down to Gimmelwald. A stop off at the water fall between there is lovely and each time I have done those hikes, we run into much less people. You have the typical Alps scenery but then your hike takes you through a pretty dense forest as you come down into Gimmelwald. Conversely, the hiking from First to Schynige Platte (consider seeing Lake Bachalpsee from First as well) takes on a more dry, almost plateau landscape with Alps and lakes in the distance. Each of your hikes will most likely take much longer than you think as you will stop and take in the views throughout.
To your question on rainy days, more specifically, I find that I tend to hike the valley from the lift station below Gimmelwald next to the river into Lauterbrunnen as one of my go to rainy day options. Or, consider heading into Interlaken and take that town in a bit along with some boat rides on either of the lakes. Also, be prepared for rain every day, even if the sky is a crisp blue in the morning. The weather is very variable there.