Edit (17/07/2021): I have updated this for N4 in a new post (there aren't many changes) - see here: https://emperorsghost.blogspot.com/2021/07/infinity-tier-list-for-n4.html
It is often said in Infinity "it's not your list, it's you". This is somewhat correct, but mostly horseshit. Some units in the game are simply inferior to others, like the Yan Huo with Missiles is strictly worse than a Hac Tao with missile launcher. Let me list some other units that are just bad: bolts (they are now passable with CoC and the changes to bioimmunity), Aida Swanson, Cube Jaegers, Taskmasters, Guijia, Takael (the symbiobug guy). You never see these units in winning lists, and I would almost say that you don't see them period. The same can be said of some factions: they just suck. Inversely, some units and factions are simply outstanding. I won't write a list of the best units, because that would take forever, but I do think it is helpful to have an idea of the faction tiers so that newbs stop playing Morats because "it's easy". MAF is actually infinity on hard mode. Yes, there are fewer rules. There's also no hope of winning unless you are very skilful in melee. I will explain my picks.
Note that some factions in tier 2 become tier 1 when the mission mix/meta is right. For example, Hassassins is nigh-on unbeatable in ground control missions when piloted by a good player on tall terrain. I would also say that the difference is only really noticeable between tier 1 and 3/4. Tier 2 against 3 is totally fair. Tier 1 against 2 is also barely noticeable. Tier 4 is always noticeably hard to win with.