I've had 8 Synology NAS products and it would be my first choice. In terms of off the shelf (ie not freenas/homebuilt) Qnap would be my next preference.
The two Dlink experiences I had ended up with them returned due to basic failures such as FTP not working.
I've had four Netgear Readynas models. The newer models might be better but all four of mine had issues. I have one still in use for backups. I have it scheduled to turn on/off. However if I manually shut it down it will not auto start as per the schedule. It also sounds like a vacuum when it starts up or gets hot.
The only issue I have had with Synology is for a period they removed a specific (FXP) feature but have since brought it back.
There are some shiny features with Qnap that does make them appealing but I would probably go with Synology based on previous/current experience.
In the past when I've needed help from support it has been excellent.
I would recommend that whatever NAS you might go with that it is best to get an Intel based CPU model. Ive noticed that quite often (with Synology) that many apps/plugins are available for Intel models but it gets a bit mixed with other CPU based ones. Third party apps are generally more Intel oriented as well.
Depending on your needs I would avoid the J series as they are the low powered/spec ones. They are great as file servers but if you are after extra usage (apps, plex, etc) then you might have problems. The lower spec ones seem to lose support the fastest due to more OS/app cpu and ram requirements with each new generation/year. eg I have a DS1511+ that still gets the latest OS updates. However my older "non supported" ones only get critical security updates but cant use some of the newer OS features or apps.
Also look for one that has at least 1GB ram and hopefully with the option to add another stick of ram if you might need it in the future if you are likely to be adding a number of apps.
Ive also found a good number of third party apps for Synology such as filebot (https://www.filebot.net/forums/viewtopic.php?f=3&t=1802) and there are others such as those at https://synocommunity.com/.
In terms of Plex, the DSxxxxPlay models have hardware transcoding but there are discussions on the plex forums asking to make use of this function. To my knowledge this has not been done yet so its still CPU processed. If you need the NAS to do transcoding I would not expect too much from it so if this is a must check the CPU limitations first such as 1080p/5.1 or x265 files.
Synology has a Demo site (https://www.synology.com/en-us/dsm/live_demo) for its next OS release which can give you a feel for it.