The good news is, at its heart, choosing your team is pretty simple!
 A successful deck will have three elements:

3) Ways to make sure 1 and 2 happen when you need them

Now, truth be told - you don't necessarily have to explicitly cover both 1 and 2.  If you do one or the other well enough - say, a deck so fast it wins before your opponent can do anything, or a deck that locks down the opponent so completely that they can't contest what you do - that'll suffice; 'the best defense is a good offence', and vice versa.  Overall, though, you'll need to have thought about all three of these points.

A Good Offense...?

I find it's usually best to figure out how you want to beat your opponent first, and build the other two out from that based on what fits.  We've got plenty of options for how to beat you opponent.  I've seen wins happen with tactics ranging from a steady barrage of unblockable attacks, to direct damage, to swarms of cheap, easily fielded characters which overwhelm you opponent's defenses, to concerted assaults from larger characters.  To start, you'll want to pick one you like, that works with the cards you have; for example, if you're looking to deal your damage with unblockables, maybe you'd select Higher Ground Angel and Canucklehead Wolverine.

The big question I ask myself when trying to pick an attack subteam is 'do I think I can get these guys doing their thing in a timely fashion.'  I'm personally sort of cautious about hinging a strategy on a 7-cost card, and even a little reluctant to do so with a 6 sometimes, because I know that there's almost no chance I'll be drawing them before turn 5, and if energy rolls don't go my way, or I need to field some characters to stop a rushing enemy, it'll likely not be until after my third die bag refresh; that's a lot of time to let your opponent not have the pressure on them as hard as you'd like.  I'm similarly leery about having a lot of expensive-to-field characters at once, because even ignoring trying to buy more dice, if you get a couple of expensive characters at the same time, you may well wind up having to discard someone your strategy really wants out.

The Best Defense...?

I find that once I have an offensive strategy, it's a lot simpler to pick some defensive cards to fill the gaps, because I can identify what will give me trouble and plan accordingly.  If I'm planning a cheap character rush, I may be worried about Charles Francis Xavier Professor X; If so, I can take Jack Deadpool or African Princess Storm to make sure he doesn't stay around for long.  Maybe I see I'm running very heavy on Fist characters, so I might want to look into ways of dealing with Thor should he come up.  Or perhaps I'm running a lot of characters who I won't want to attack with - Flame On Human Torch, Angelo Fortunato Venom, etc.- and I need some way of dealing with Mr. Fantastic or Phoenix's global, so I'd naturally consider Distraction for one of my basic actions.  If nothing really pops, you definitely want one generic, reasonably accessible way of handling threatening dice, and preferably two, so that if one gets tied up via Loki or something, you still have another option.

Gotta Go Faster!

Presently, the best way to make sure your strategies go when you need them largely comes down to drawing extra dice.  More dice rolled equals more energy equals more dice fielded and bought; on top of that, more dice drawn means you're cycling through your dice pool faster, meaning you get what you want sooner.  Which one works best will again be informed by your previous picks; for example, if you're running Flame On Human Torch or something else with a damage-dealing effect, Quasar Nova might be a good fit for some mid-game die acceleration.  A squad with a lot of more expensive characters might want to consider Brimstone Biker Ghost Rider, as you can buy a big die and then field GR, and skip having to wait for your bag to empty to play them (though this does take a lot of energy - Ghost Rider isn't cheap to field).  A lot of squads could benefit from the versatility of Norman Osborne Green Goblin, who will usually field for a net gain of energy, or can help you prune sidekicks out of your bag, but you may want a Gambit instead to hopefully get characters straight out onto the field.  Using these resource-amping cards right can seriously help your strategy, and I think doing so is likely to be a topic for an upcoming podcast - if not, I'll certainly get some more in-depth thoughts in on them in the future.

Paring Down for Surprises

With around four or five 'offensive' picks, two or three 'defensive' picks, and one or two 'accessibility' picks, that should pretty much round out your eight for a constructed game.  Choosing a base four for a sealed deck game is a lot more difficult, and I think mandates a really tight offensive team that probably leans pretty heavily on using one of the resource amp characters as part of the attack strategy, and has only one slot devoted to a (probably generic) defensive pick; you can't be sure of what you'll get out of the boosters, so you have to be pretty sure you can win on the strength of those four alone.  Getting a good pull will only help your cause, if so.

With a team selected, and an idea how you want to use your team, the next step will be figuring out what to buy, and when...which is an excellent topic for my next article!  Join me Saturday for Intro to Dice Building 2: The High Cost of Dice, where math and probability may rear its ugly head.