Wii Fit, is the best trainer, and probably the best to start with. If Wii Fit can't motivate you then none of the others will. Hopefully the followup title will launch later this year.
My Fitness Coach: This is more like having a personalized DVD fitness video. To get the most out of this title you'll need other equipment such as a step, balance ball, dumbbells, heart rate monitor. Every workout is different, and can range from 15mins to 75mins (in multiples of 15mins), and can have a different emphasis, you get to choose. You can also up the difficulty with ease. Oh and it's cheap! I really hope UbiSoft are working on a follow up, I'd certainly buy it.
Family Trainer: For training it's not in the same league as Wii Fit or My Fitness Coach. It is however great fun, and you can play vs another player on some of the games. Don't get me wrong you can work up a sweat using this, and it is great fun for all the family, but it's not a serious training and fitness program.
EA Sports Active: Not released yet, so I can't really say. I have my copy pre-ordered though.
Jillian-Michaels-Fitness-Ultimatum-200... Apparently this is one to avoid. I don't have this, but all the reviews basically say it stinks!
If you can afford it, I'd get both Wii Fit and My Fitness Coach(cheapie), the variety will be welcome.