"Not even" the first one? ME1 was basically cool ideas with very poor execution. Pretty much everything about the game, be it the writing or different gameplay elements, was either shallow or hollow. Or both. In ME2 they refined it a lot and delivered a tighter, much better game in pretty much every department. Scrapped a bunch of stuff that didn't really work in the first game anyway and would likely never work, and improved vastly on the things that were promising. As a result ME2 was a competently written, much more tightly designed and enjoyable action game with some minor RPG elements to spice things up.
ME2 is neither a good RPG, nor a good action/shooter/adventure and does not improve upon ME1. At most you can say it's a "sidegrade".
On the gameplay side you a bland cover shooter:
1. The introduction of ammo; it makes no sense in the world of ME and even the writers know that considering their lore reason for the change is so stupid and they know it. Talks about "DPS" even for stuff like snipers for example.
2. Ammo also exacerbates the cover shooter issue
3. Introduction of "heavy weapons", from grenade launchers to reaper tech to blackhole guns that all use the same "energy cell" ammo.
4. Useless Classes; on the higher difficulties you essentially need Miranda with you at all times and others who can use warp/overload to strip the multiple layers of armour on every single enemy in the game. Once the armour is stripped, the enemy dies way too fast for biotics to work at all so Adept doesn't work. Remember the days you could send fully armoured Krogans flying into the air with biotics?
5. Gameplay changes added tons of narrative issues & plot holes. ME1 was pretty good with narrative cohesion in almost every aspect (I'm not including dialogue options in that). For example, the new droids & heat sinks being used in areas that shouldn't have access to them (i.e. Jacob's loyalty mission)
6. Global cooldowns, not just the one used like in ME1 which negatively affected gameplay as well
ME1 combat on highest difficulty rewards positioning, ability use and speccing yourself & party members
ME2 combat on highest difficulty is stripping 2-3 levels of armour waiting for ability cooldowns on enemy after enemy and conserving ammo
ME3 combat is ME2 but much more refined, won't go into specifics here but the fun, action combat they're clearly going for is at it's peak here
The only improvement I can think of is enemy AI, and when the level design was at it's best, it provided some decent thinking instead of mind numbing cover shooting. Tali's loyalty mission when you meet Kal Reegar & the final fight on Horizon come to mind. However, the quality there is still bogged down by the cover shooting mechanics that haven't been ironed out. In contrast, level design in ME3 was consistently good to decent all around apart from some misses like the final mission, without being as bogged down by monotony.
It reminds me of the Witchers, the second game does a sidegrade/downgrade into a new system where both have their flaws (instead of improving on the first) and then the third games combat is much better than the second (for what they were going for).
On the narrative side, ME2 has a retarded plot that also destroyed any progress in what's supposed to be a trilogy and has some of the most retarded story moments in all 3 games. There are no sidequests apart from random shit you find on random planets that are also shallow. The loyalty quests are just checklists & daddy issue drama. It also goes into the "chosen one" thing where Shepard can do anything without an actual reason of why he can do it in ME1. Improvements came only in the character department where you get more personal instead of most characters being information dumps for their respective races (ex. Wrex was the only exceptional character in ME1 imo).
With all the downgrades/sidegrades, and a few actual improvements, I don't see how ME2 can be "an improvement in virtually every department".
