I've actually grown to really enjoy the game play in the original Mass Effect but that's after playing through it many times and now I have a solid grasp on what it does well and what it does poorly, and I know how to manage the clunky inventory (this is on PC, so it's slightly better than the consoles but still not good). I also liked the Mako but I didn't like the crazy vertical terrain you had to navigate on the uncharted planets. Story Mako missions played fine for me at least on PC (I played the Xbox version once when it released but my memory of how it played there is fuzzy).
The second game improved gunplay but at the same time made biotics boring. Global cool down slowed everything down and the cool downs themselves were too long. The biotics I found most interesting, throw, lift, and other force based powers where useless against enemies with a Barrier, Shield or Armor and once an enemy was stripped of those, there was no point to using lifting or throwing them. They were already virtually dead. Also, I hated that they added the heat sinks and their ridiculous explanation for them. They could have kept the cool down mechanic and used the stupid heat sinks as a quick but not mandatory way to force the weapon to cool down without making stupid nonsensical lore changes to make players, used to reloading, more comfortable. Fuck the inventory system, like a lot of things that could have been improved, they decided to just cut it out. I'll take the Mako over the Hammerhead, which was just kind of boring to use.
Third game was a legitimate 3rd person shooter and they improved the biotics some over the second game, with opportunities to shorten cool downs drastically and more explosive power combinations but I still prefer the original and it's individual power cooldowns. Gameplay-wise, I think the third game was good enough to support its combat only multiplayer feature. It was much better than I expected.
Story-wise, I don't think I want to type out all my thoughts on the universe and story again.
I'll just use this lazy shorthand:
ME1 >>>>> ME3 minus the ending > ME2 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>ME3 ending