A Cycle Analyst gives the most info plus it has a number of other features that are great if you use them. For several reasons I never owned one:
1) I exclusively have geared motors so the speed function does not work for me,
2) I've only ever owned one controller with a direct-connect CA connector (and I removed it)
3) They are pricey
4) When I first started E-biking I mounted a Watt's Up meter on the handlebars. I liked watching it and with a small battery it was useful. Once I got bigger batteries I didn't need to closely monitor my power consumption. I do like to collect ride data so I toss a meter in with my battery and record the trip info on a log sheet at the end of a ride.
5) It can't be easily moved around. I use the meters for various bikes and when recharging batteries so I use it on and off the bike.
Here are the wattmeters I own:
Top is the Watt's Up meter. The first wattmeter I owned was a WU meter. I blew it up and bought another right away because I can't do without it. They run $50-60. I like the small size and accuracy. It also is the best sealed of the bunch. I ran a bead of superglue around the edge of the display on my first one that was used out in the open. The display can flake out if it gets hot so I no longer carry this one in the trunk bag with the battery.
The middle one is a Turnigy wattmeter. These came out a couple of years after I bought the WU meter. I bought TWO of these for the price of a WU meter. Unfortunately 1 of the 2 calculated Watt-hours incorrectly so I tossed it. So was it a bargain and what if your ONLY wattmeter is inaccurate, how would you know? They do have adjustment pots inside for voltage and current calibration, a nice feature, but it didn't help with Wh. The case is vented and sort of rubberized making it robust. This is the one I have been tossing in with the battery for trip info.
The last one is something I just got today from amazon for $15 delivered. The case is metal and is definitely not waterproof.
For the price I would probably recommend the Turnigy or one of the many clones available now.
-R