I started playing when the open beta was first released but after a month I got to a point where there wasn't much else I could do so I stopped while keeping up with the updates through my favourite content creators. The most recent update was so big I just had to try the game again but even with all the new heroes and mini's my favourites haven't changed.
Countess used to be the most fun hero before and I can confirm she still is, although now she has good competition like the Skeleton King who would be a close second or Queen Archer who does much better now with the recent changes than she did before.
How I usually use Countess is I place her one tile below where my frontline is so she doesn't get targeted, but if you're on a energy charging tile map you can get away with putting her in the frontline assuming that's where the tile is. She teleports to the lowest hp target now instead of the farthest one so she's guaranteed to instantly latch on to a squishy backliner although the Witch can have her skeletons soak up the damage (this however grants the Countess a stack for her ability so I wouldn't recommend it as a counter for every scenario.)
The Dart Goblin is my absolute favourite mini with it's insane dps ramp-up as the round progresses. It gets even stronger with upgrades since they give it a hitspeed bonus which let's it build rage stacks even faster. Some notable counters to it are the Archer who can hit him if you put him all the way in the back so I would recommend putting at least one unit behind it to soak up the arrows, and Shield Maiden who deflects all it's damage causing it to melt itself. For the Shield Maiden it can use it's third upgrade to still damage it when it's getting damage reflected back at itself but it will still end up doing too much damage to itself a lot of times making the upgrade not that good sometimes. You just have to get lucky on the Maiden's super timing.