I only cycle to power on a static bike as I prefer to cycle to feel when outdoors but... a powermeter that doesn't give you reliable feedback is worthless (even wattbikes are temperamental)
Also some powermeters (SRM) dont allow you to replace the battery without sending it in for a service - watchout for hidden costs.
Id only recommend Infocranks by verve cycling.
The standard recommendation for all triathlon purchases is to check https://www.dcrainmaker.com/
He has a pretty extensive power meter section.
Have a think about which style you might want. Popular units are Powertap hubs or pedals, Power2max, quarq and stages. SRM used to be the gold standard but are crazy expensive.
Dc rainmaker likes the powertap (p1) pedals and if you've got multiple bikes it's an easy swap. They also come in single sided or double sided with single at half the cost.
Powertap hubs as mentioned by Tony, good if you train and race on the same wheels, these guys normally the cheapest, measure total power output so no left crank only measurement issues.
Left crank only like stages, these are also cheaper but will only measure the left-hand side so some people have issues with that. Team Sky use them, not sure if that's just thanks to sponsorship or because Sky feels they are the best.
Then you have spider crank based which have come down in price but cost more than the previous. They are popular because they measure total power like the hub style, but you can change wheels. The downside is you cannot move it to another bike easily thanks to different bottom bracket standards. Power2max, SRM, quarq.
But totally agree with Edward, Dc Rainmaker is the best source to find all the info you need. I've got quarq dzero which I'm happy with.
Thanks for all the really useful information guys - I really appreciate it. Looks like
I'm going to be either going for the powertap hub or p1 (both sides). I now need to get this past my wife....any tips on that! Haha