Well, we have had poor experiences with Megaflo. Leaking immersion heaters due to poor welding, and rattling combi valves. The bubble top only lasts a year if the unit gets through a fair bit of water. And until the factory was refitted, the quality of output was very variable. The floating baffle patent doesn't seem to do anything at all. The claimed 50+ litres through the valve is a nice gimmick, never to be seen by a domestic customer on Thames Water.
We like the Ariston because it comes from what is billed as Europe's most modern unvented plant in Belgium, has class leading insulation and is s/steel. The old glass lined units are still made and perform better in certain environments but we always use the s/steel Primo in the South east, where we are based. It is also part of the DualStream integrated package so we can give a customer an upgrade to a factory spec DualStream at a future point if they so decide - which makes 50l/m a distinct possibility.
The OSO is a very heavy cylinder but comes restricted to 2.1bar if used as a bubble top. If you want a 3bar version they have supply a spec order with a different combi valve. The electrical connections are a bit flimsy. Another plus point is you can get a DualStream version.
Finally, the Vaillant uniSTOR is a little beauty and addresses tight installs by having most of the connections on the front. It also is WRAS approved for thermistor control from a Vaillant eBUS system, giving much better control and economy when used with an ecoTEC boiler.
I should have mentioned Viessmann's unvented range, which is expensive, massively constructed, can be thermistor controlled, is difficult to order because of the different installation options and the installation guide is very Germanic. Typical Viessmann really. Very heavy, not for the solo heating engineer.