I think with this trilogy the distinction of best to worst isn't that meaningful, since if you're a new player it's recommended to play them in order.
But 2>1>3.
2 is objectively better than 1 in most technical regards (graphics, gameplay) and the storyline is a bit more engaging. 1 has a special place in my heart because it introduced the world, was more open world and I enjoyed the planet exploration->mako gameplay loops. I really enjoyed the over-the-top squadmates in 2 that really felt like a mega-wealthy person hand-picked them instead of the typical party-RPG where you find a bunch of interesting losers who grow with you.
3 was a great game, especially if you enjoyed the multiplayer as much as I did, and it had technical improvements over 2 while having a lot of great story sequences. However it was heavily marred by its ending and a few elements interspersed throughout the game. I've heard the updates address them but the ending is fundamentally flawed and by the time ME3 came out, Bioware's talent pool was so degraded they couldn't really finish the series properly.