Which Minions/Mercs

I know I would have avoided a lot of models on my table of shame if I had asked this. I am definitely not the best person to answer this, but my two cents (ordered by frequency in lists) are as follows:
1) Swamp Gobber Chef - he costs one point, and is the cheapest fill in the game. Who cares if he does at scoring the enemy flag for one turn?

2) Swamp Gobber bellows crew / feralgeist - 2 point models that can score round zones and flags, respectively. Back in the old days I made use of the clouds, but that is much rarer these days. The Feralgeist is incorporeal and hilarious if you manage to trigger his rules, especially with a durable enemy beast.

3) gremlin swarm - it's an even more survivable feralgeist, with rules to upset everyone that isn't playing undead models.

4) Lynus&Edrea and Lanyssa - again, utility and you can capture zones and flags easily. Lanyssa gets special notice in Storm of the North where you can plant a snow drift to trigger her prowl while she camps a flag.

Everyone else is lower priority for me, but that has to do with having a strong disinclination to buy minions after buying a Totem Hunter (really cool, but hard to pay for), Raluk Moorclaw, the drunk suicidal Ogrun whose name I've forgotten, and others that I have not fielded often at all.

Saxon Orrik and Alten Ashley are both with considering, they just haven't quite pushed me to buy them yet.

Awesome! Just curious what’s the typical “play” with Alten, or the Efaarit? How do use them correctly? Also, does totem hunter see any play?

Always useful for scenario play: Swamp Gobbers, Feralgeist, Chef, Gremlin Swarm
Very helpful for the current meta: Eilish, Bog Trog Mist Speaker, Lynus & Edrea
Nice utility to have for the right list: Lanyssa, Croak Raiders, Wrong-Eye & Snapjaw
Generally just fun: Longchops, Efaarit Scouts, Razorback Crew

Trolls have reasonably good access to shield guards so they've got some competition, but farrow valkyries are totally worth their weight especially with a debuff caster like grissel1

I still want to try Hutchuk in a Storm list that already has bears. When a big hammer works so well, how much better will a BIGGER hammer work?

I think Arcaux has hit the nail on the head, although I’d add two caveats. One is, if you’re playing in the Power of Dhunia theme, the lack of available units can mean you might not have a lot of attacks, so a 10-man minion unit like farrow brigands, Nyss hunters or croak raiders can be helpful. But only if you’re playing Power if Dhunia anyway. And even then, a big unit is far from essential, and a lot of the top lists don’t include one.

The other is that a lot of the advice here is for building an optimal tournament list (and for that, it’s very good advice). However, if you think a model like the totem hunter is cool, go ahead and add him to your list. He may not be the most out-and-out powerful choice, but he’s a lot of fun on the table, and the model looks fantastic!

The only 2 minion choices that I’d say are almost essential for you collection are the gobber chef and the swamp gobber bellows crew, just because they’re great point fillers. After that, the stuff in the list Arcaux posted is all solid.

Proven merc/minion solo/units that make a lot of my lists are Lanyssa and Nyss Hunters.

Nyss with Kolgrima's cursed fate are awesome against high armor-single wound enemies. I like them in my PoD lists with the ol'lady.

Lanyssa in this heavy terrain edition can easily hide in forests, while using winter storm to nullify enemy pathfinder. Kolgrima's cloud spell can give her stealth as well.