IMO, the games are pretty much on par but for different reasons. I enjoyed the combat in ME3 the most, it has some of my favourite levels (Tuchanka, Grissom Academy, AY Monastery) and I loved the urgency of the storyline and the way the main sidequests tied into that. I also thought there were some fantastic moments between Shep and crew, and between the crew themselves. But ohhh, dat ending

And too much autodialogue. And Insanity is a bit too easy.
ME2 had the best cast of squadmates, and the most interesting dynamic with Cerberus, a few nice non-combat missions, and the Hammerhead which I found a lot of fun. Plus the suicide mission was incredible. But weak overall storyline, antagonists (Collectors) I wasn't very interested in, horrid 'Mission Over' screens at the end of levels, and boring mini-games. Also, more streamlined than ME1 or 3 in terms of skill/power progression imo.
ME1's main quest was the one I found most enjoyable overall, and had the best sense of exploration, and I loved the Mako. Still my favourite skill/weapon/power progression of all three games. It also gets nostalgia points for being the intro to the trilogy and the universe. But head-smackingly frustrating and clumsy combat, some really boring planets to drive around, and re-use of assets almost as bad as DA2s. And an inventory that was painful to manage (this coming from someone who loves a bit of inventory shuffling).
So for me they're all the worst. And the best.