You can't really just slap the best characters onto a team slot in any position and expect them to really work well (maybe you could). One thing that I feel that is crucial when picking a team is a leadership that benefits all characters. That's far more important than team bonuses (which I assume you mean by synergy) by the time you're getting ready to face fully maxed teams.
For example, Loki is undeniably one of the best, but putting him as a leader is useless (50% mind damage resist?). Not to mention the clones that make him so good start out on cooldown, so you'll have to wait a bit.
Don't forget about ISO-8 sets, also more important than team-bonuses. Pick what kind of build you want on a certain character and decided which set suits them the best. Once that is done (may require experimenting), slap some high level ISO-8 to get the most of the stat boosts as you currently can.
Of course, there's gears. You will definitely want to rank those higher than they are now (will take lots of patience) to be capable of taking on difficult opponents.

My Timeline team consists of She-Hulk, Silk, and Captain Marvel (with uniform). She-Hulk's leadership helps with almost any all-male teams, and if not, switch to Silk's. Captain Marvel is there to handle the energy-based enemies or physical damage immune ones. Silk is there to subdue any hard opponents and protect herself from them (Loki if he's in the last slot of the opponent's team, for example). They are all definitely not invincible, but they can get the job done.

That's probably what the top 100 Timeline players do. They pick the best ISO-8 sets for their characters, give them high level ISO-8, pick a good enough leadership, max out all gears, max out all skills, and don't forget about uniforms. Synergy is helpful (it can let you cover all types or give extra bonuses), but it's not an absolute necessity. There's no universally "best" Arena team.