Which (combat) playstyle is most fun?

Except for pure biotic, I've done two of each class, one for each Ryder.  When I do the second pure biotic, that is the last SP run I intend to do.  Except for Explorer, all the classes have been played on insanity.  I tried profiles ... even know how to set them up and save them so they work ... but it makes it too easy.

Typically, I get three of the four weapons I prefer in loot drops, I'll manufacture the fourth at level III, along with the chest armor, for the side missions ... that generally carries me through to level VIII, where I build the weapons and armor to take on the architects and the second run through Meridian.

Originally, when I thought there were going to be DLC, I saved up research in anticipation of the DLC getting me to level X, where I would manufacture my end-game weapons and armor. As it turns out, level VIII is end-game.

All my runs have been with fresh starts. I've alternated Ryders, my final run will be Sis Ryder Adept ... and she will make all the things go boom.

I've tried to set up my two runs in each class, so that they are not copies of each other ... so some overlap in a power, but generally, run different power schemes and play styles. Overall, engineer was probably the easiest, though vanguard might argue otherwise. But generally speaking, by the time I get to level 25, I have enough skill points in passives that things are not too tough. For the Adepts and Vanguards, by level 53, I have all the passives that are relevant, plus all three powers maxed (same point my squadmates max their powers). For the other classes, I hit that point at level 58. Any levels beyond that, I apply points to additional powers that I might use for that character class.

A typical run takes me to level 65, had two at 62 (bugged side missions wouldn't spawn), one at 69 and one at 71. I can't really explain the last two ... obviously shot a whole bunch of stuff that I didn't shoot in the other runs.

Getting back to OP's question, which one was the most fun? For me, infiltrator was the least fun ... in a lot of ways, easier than the others, but sometimes tedious. Explorer was fun, but it ended up being a lot like the Sentinel ... so lots of fun, but kind of duplicative beyond the bonus passives. I had a blast with the Engineers and Soldiers (EDI says I need to work on my joke delivery). I'm saving the second Adept run for last, because that is probably my favorite. She'll be a cross between the Asari Adept and the Krogan Gladiator ... pound all the things and make all the things go boom ... and if that doesn't work, shoot them.