During my what? 3, 4 years, of playing ME I believe I've only come to around maybe two boss battles? Minus all the DLC, of course.
One being Saren. One being the Reaper at the end of ME2. Hard battles? Sure, we've all had those. Those are about the only two "boss" battles I can really think of. And both of them were absolutely necessary. I loved fighting Saren. Almost as much as I loved the cutscene where the Normandy blew up Sovereign.
Those were the boss battles at the end of the game, but there were plenty: The Reaper on Rannoch (ME3), the Thrasher Maw on Tuchanka (ME2), the two encounters with Kai Leng (ME3), Shadow Broker and the Spectre who worked for him (LotSB), rogue VI (Overlord), Donovan Hock (SM), the Thorian (ME1), Saren at Virmire (ME1), Matriach Benezia (ME1), EVA (ME3), etc.
You really enjoyed Spider Saren? I found it to be incredibly underwhelming, and short. I killed him in less than thirty seconds on Insanity. Wasn't much of a fight. The Human Reaper I killed just as fast on Insanity. Neither fight was memorable to me, personally. I think the DLCs and the various boss fights littered throughout ME3 were much better executed.
Personally though, I'd like to see BioWare do something more interesting than a traditional boss fight. One of the most memorable boss fights in gaming was when Link encountered his doppelganger in OoT. An incredibly epic and unforgettable experience. It would have been interesting had the Reapers constructed a Husk or even a mini-Reaper that mimicked the capabilities of Shepard giving him all of your abilities. Then build a fight around that truly pushing the player to his/her limits. Battles are also more interesting when the actual environment is changing throughout the course of the battle. That was not the case in either of the two final boss fights.