Nemo - I say being the FIRST ride of the day when the park opens.
To add to that...if you have Magic Morning and you want to use it on Nemo instead of Fantasyland, do it the first ride on that day.
If you do NOT have Magic Morning (or you're using it on Fantasyland), be sure you don't try Nemo first thing on a MM day. The line will be full already.
So if you're not using a MM on Nemo, be there early early so you can be one of the first on Nemo on a non-MM day.
MM days are the S and T days...saturday sunday tuesday thursday.
TSMM I don't know. When I was planning for our December trip, I swear everyone was saying to skip the morning line and go later in the day. We did so and encountered 40 minute waits*. Then I got back and found everyone suggesting that the morning is the best time! Since I read here all the time I was a wee bit confused about when that changed.
*40 minute wait at TSMM isn't bad at all, IMO. I love the way they've designed the ride queues for both Nemo and TSMM. Nice and wide, and brilliantly designed switchbacks. And it's fun to watch everyone's faces as they realize that a 40 minute wait on TSMM includes going into a building that you didn't know was there until you got there...and knowing that that is exactly how YOU looked just as you noticed the building too, LOL. Even our 4 year old doesnt' complain about Nemo and TSMM lines, b/c they are just so pleasant in our opinion.
It does, however, take that time off of the parks, but for us, TSMM was worth it, and one day we spent a total of 70 minutes posted wait time for two rides of TSMM and it was worth it.