The Nokia 5210, What I remember most about this phone is dropping it from 2 floors up and when I went down to pick it up, it was still ringing!
The Nokia 5210, released in 2002, with its hard shell was pretty indestructible, making it one of the most durable Nokia phones till date. they were also interchangeable and easy to replace.
It had an in-built thermometer sensor that read the temperature of its surrounding, pretty neat for a dumb-phone(though I don't think we can call it that now), I don't think any other phone has a thermometer sensor apart from those with the Accuweather app!
The Nokia 5210 was pretty water proof (to some degree), I mean you could drop it in water, but that doesn't mean you leave it there now.
It also included the Nokia original game snakeII (an upgrade from the original snake) it just had a more real life looking snake in it.
The way the 5210 casing opens up paved the way for some of the phone casings we see today.
FUNFACT: The Nokia 5210 was featured in the movie: Blade 3 (starring Wesley Snipes)