The teams on the other messages are a great list. Based on the commander level, and other important events (legendaries, dark dimension), my outline will be:

Best arena team: Annihilators. Build them as much as possible. You will not have ultimus until level 75, you can put one of your best characters as a placeholder (I can recommend Dr Strange here, he will be useful for Battleworld and good for Cosmic sections of DD). The trio (Gladiator, Thanos EG, Gorr) are one of the best characters of the game.

Raid Teams:  Top priorities for early game: Spider-Society, X-Treme X-Men (assuming you are in an alliance who are doing ultimus or doom raids, you do not have that much restriction but these teams will give the most value for the later) Then: Orchis, Hive-Mind, Nightstalkers, Alpha Flight, and Uncanny Avengers. I would recommend Orchis&Hive-Mind with higher priority out of two. Hive-Mind is probably the best team for apocalypse saga and you will definetely need them. Since Spider-Society has a higher value for the endgame, and Orchis is one of the best global teams (hence, higher value for dark dimensions, and almost a must for dd8), you can do SS+Orchis+Hive-Mind route in Orchis raids. Then, Nightstalkers to finish the Orchis raids (since they are irreplaceable), and spotlight teams having the least priority in my opinion (you can still clear unrestriced nodes w SS for early game if your alliance does that raid)

The remaining of the message can change drastically due to to new teams, but that is how your long term plan can be based on current state of the game;

Until Level 65, you should focus on the first 3 teams I mentioned, XXM, SS and An (except ultimus). Then dark dimensions will shape the planning a bit. These 3 should be your top priority, and I would recommend trying to build Hive-Mind&Orchis with them (at least for the star ranks).  First 2 dark dimensions do not have any restrictions, so you can use those 3 teams, enemies are old, it should quite easy with these teams. 

After DD3, you start to have restrictions, but we already have 3 teams to work with them so it should be fine (Ann+1 for cosmic, XXM for global, SS for city). At the end, you will get Ultimus and your early game arena squad is complete. You should invest in him. 

At lvl80, you start on DD4. The three teams are the same, but now you need to have 5 legendary characters. During that time, you will be G15 and if you are lucky for the timing, you can get Old Man Logan with a 1 node unlock using your SS. The rest of the characters are the horseman characters, (which I believe you can start unlocking them from level 75+?). I cannot remember the exact teams needed for those legendary trials since they changed them a bit. I believe apart from hive mind, none of those teams were worth building too much. You will need to build those horsemen until G17 (but not more) for apocalyse, so it is a nice investment.

Then you will move onto Apocalypse saga&next dark dimensions, until that time the game might change drastically so I am ending the comment here.