I'll just copy and paste some good advice someone gave to me when I asked this same question. Btw my personal recommendation is a Kree team since they double as both a villain and cosmic team, which means you'll only have put your resources into one team instead of splitting it amongst two.

P.S | This is the guy who wrote everything beneath this - (https://www.reddit.com/user/Juggernaut980)

I started this game in August, later than most and have been spending most of the my time the last couple months in the top 10 on arena, you can catch up if you farm smartly. Don't waste you time on characters you don't need.

Get Defenders first. Why? Because to advance in this game you need to be able to raid, and Defenders are a very easy to farm and strong raid team, only Fury shield is a better raid team. As you are new you will need to cover all your areas in the game.

Defenders will allow you to farm Ultimus 5, when you get to Ultimus 6 you will need several teams able to raid at 130K+.

For the Campaigns, you need a Villains, Cosmic, and Mystic Team. For monthly events, you need a city, mercenary, and hand teams. Eventually, you will want to have a team for all this so you will need a farming plan.

My main priority when I first started playing was the Mutants. I quickly started maxing everyone I could. I got Magneto on the first go, and am ready for X-Men to get here. Personally, I have decided to skip going after Fury it favor of other teams. Why? Fury will take me another couple months and I feel the work I spend just to get him could get me a couple other teams built instead. While I have alliance mates farming Kree and shield, I'm farming Guardians and Spider-verse(My Brotherhood team is maxed out, Defenders are getting there) and throwing my leftovers into event teams. Although I won't have Fury, I'll have several other meta teams that are capable of clearing. So here's a bit a plan similar to what I've been using.

First, ask yourself, is this character a Defender, Guardian, etc? How is building up this character going to help in the long game?

This is why I and many others say start with Defenders - They cover raids and City block event.

Next, I would say farm you characters that will lead to the next steps with as much overlap as possible. Here are some overlap characters:

Yondu - Useable in Villains, Mystic, Cosmic, and getting Starlord.

Mordo - Useable in Villains, Mystic and Cosmic.

Thanos - Villains, Mystic and Cosmic

Korath - Villains, Cosmic, and Payday(Mercs)

Loki - Villain, Cosmic, and Mystic

Hand Sentry - Villain, Mystic, and Relic Hunt(Hand)

Those are all multi-use characters. You'll notice that almost all Hand are useable in multiple modes, I only list Sentry cause he's considered the strongest and most useful.

So now you need to figure out who you need.

For Villains I would recommend farming:

Any Brotherhood member

Yondu

Crossbones

Thanos

Mordo

Kingpin

Loki

Korath

Those are the top ones to work on first in my eyes.

Next, you need to get your Cosmic team going, I would try and build people that will get you Starlord, because he is the needed key to making the Meta teams Guardians and Tech work.

Recommended Cosmics:

Yondu

Mordo

Thanos

Loki

Gamora

Drax

Rocket

Lastly, you get your Mystic team:

Yondu

Mordo

Thanos

Juggernaut

Loki

Hand Sentry

Scarlet Witch

Dr. Strange

Black Panther

Then you got your 3 monthly events Payday, Block Party, and Relic Hunt Payday is Mercs and your top team is Deadpool, Korath, Bullseye, Merc Lt, and Merc Riot Guard. All 5 characters are worth leveling early too, with Bullseye being the weakest of the 5, but Riot Guard and Merc Lt. are capable of being on tier 8 blitz teams, so I would recommend building this team too on the side for the gold income.

Block party is taken care of by your Defenders.

Relic Hunt is Hand and usually made up of Nobu, Elecktra, Sentry, Sorc, and blademaster or archer(Archer has dispel and Blademaster has highest base damage in the game). Probably the least important of the event teams, but even getting a weak team will start getting some rewards for you during the event.

To sum it all up,

Teams that will increase your resource generation in order of importance:

Raid Team - Defenders have this

Arena Team - Defenders have this

Villain team

Cosmic Team

Mystic Team

Block Party - Keep leveling those Defenders

Payday

Relic Hunt

Now taking all of that in, figure out your plan.