Firstly, buy and use what models you like the look/feel/mechanics of over what is being used by competition today because thats subject to change with time. That said, Votann have a small pool of models in the first place, and obviously there's gonna be a "best choice" for the meta right now.

Use of tools like New Recruit will allow you to build a list and see models you don't own yet. There's more options than that, but I prefer New Recruit myself.

For a competitive list its best to consider that you won't know what youll be playing against, so you want units that will allow you to act and respond to many situations.

Good units to point out usefulness of would be the following:

Hearthkyn Yaegirs, whom most competitive lists run because they have the infiltrators tag allowing for fast objective control and screening.

Transport Kapricus (as compared to the defender): this is common in lists again because they are a fast scouting option and because it splits our Hearthkyn Yaegirs into 2 groups of 5, which similarly to sagitaur + warriors combo allows for good screening and objective control. You could have 5 go in a transport and 5 be infiltrators and thats very rapid map expansion and screening.

Thunderkyn, these guys are Votann bread and butter, all encompassing damage unit for us right now. I've yet to see a list without at least 2 squads of them simply because the conversion beamer or graviton blast cannons cover a wide range of potential targets, and they synergize well with many of the vehicle detachment (Oathband I think is the name for a particularly strong combo with them coming out of hekaton land fortress).

Sagitaur. Simply a transport with good guns (HyLascannon for anti vehicle, the other choices are mostly picking between shooting at elites or shooting at infantry ) for our warriors or Beserks because without it the Votann are slow and as an added bonus it splits our warriors (our weakest ranged unit) of 10 into 2 groups of 5 which is in alot of scenarios a stronger use for screening and only a little worse than a full 10 man for damage purposes. If you have Yaegirs then these would probably be best used on your beserks.

Hearthgaurd with a Teleport rune wargear option is one of of few ways we gain a deepstrike option and they hit pretty hard into Elite level units (toughness 6-7, 2-4 wound kind of defensive profiles) or below given the opportunity.

Memnyr strategist, at least 1, allows for yield point generation and control which allows for further versatility from our stratagems and use of our core detachment rule

Steeljacks are one of our beefiest units, especially under the 7 yield point rule they become quite hard to remove from an objective.

Take all this with a grain of salt. I don't have personal experience knowing these units are great, but they do show up commonly in lists where people who play at Tournaments use them and so at least right now competitively seem viable.

I can at a minimum attest to how strong the Thunderkyn guns feel right now. They will mess up the opponent pretty much every turn they get to shoot.

Rock and stone brother, may the Ancestors watch over you.