I have tried many of the VTTs out there now and this is how I thought of a few of the main ones.
As a first point; I don't play DnD or Savage Worlds, which seem to have the most online support, so the experience is likely to be different for those.
BattleGrounds: Works well for battlemaps, developing campaigns and using assets. You can have multiple assets for the same object/character/npc, so can have a character that is mount, on foot, holding a bow, using a sword, etc. The only VTT that I came across, that did this. It is older though and requires that you have the download on each machine and found that we sometimes got regular gateway conflicts for no apparent reason. Also made as a one man show, so support can be tough sometimes. I could build my own macros though and for standard polyhedral dice it worked fine. It also did both Fog of War and Line of Sight, but both were quite complex to do. The initiative tracker is very usable
Roll20: I really didn't like this too much, but being said, they have done a lot of development since then. My biggest complaints were that the upload file size was incredibly limiting for the detailed maps that I draw and the amount of assets I could potentially be using. It was also heavily designed for using squares instead of hexes and didn't seem to do asset facing very well. It did do fog of War, that Fantasy Grounds didn't, didn't do non-standard systems very well.
Fantasy Grounds: As said by others, it's a steep learning curve and you need to use the MoreCore variant to get the most out of non-standard systems. This is definately possible, but I have hurt my brain several times to figure out the maths to build macros for roll low systems. However, it is good for developing complex campaigns and you can use really detailed maps and assets. I am still doing some gaming on this, but needing to learn LUA and Unity to take my knowledge much further...into developing themes and module paks. Now Unity is out (have been using since Alpha). There is Fog of War, Line of Sight and dynamic lighting. They will also be able to develop into mobile access using the Unity engine too. Doesn't deal with Cards very well yet. I do like the Initiative tracker and the automation you can build into when dealing with large numbers of NPCs.
Tabletop Simulator - It's very pretty and 3D. Easy to use and build sets, but you will need to use other applications for the campaign management aspects.
It has FoW, LoS and Dynamic lighting and can use highly detailed assets. Like FGUnity it is made on the Unity engine and I am just starting to touch on scripting, and you need this to get the most out of it, like to building custom character sheets. However, there is a very large pool of talented developers doing the work of putting games on here. So unless you want to do homebrewing or are using quite rare systems there is generally something pretty good already built. It does card and dice games very well. There is no initiative tracker either, so this can be a deterent for some.
In summary; if I am playing any of the following; Forbidden Lands, Fate Core, Traveller, Tachyon Squadron, Cat, DemiGods. I use TableTopSimulator with OneNote as my campaign manager.
For tactical games, like Dragon Warriors, I am using FantasyGrounds Unity with the MoreCore rules set.