Torterra is the first of three consecutive Starters in this list. My favorite Sinnoh Starter, and my favorite Sinnoh Pokemon in general, Torterra is one of the main reasons I got back into Pokemon. I guess I should explain. Like many kids that were introduced to Pokemon in the late 90's, I eventually 'outgrew' the series after Generation II. Sure, I bought Pokemon Sapphire when it came out, but I wasn't all that into it at the time, and I only bought Pokemon LeafGreen because all my friends had it and I didn't want to be left out. When Generation IV was first announced, I wasn't even very interested. One day, out of sheer boredom, I decided to take a look at the new roster of Pokemon. My initial reaction was that some of them were pretty neat (like Honchkrow and Mismagius,) some of them were horrendous (Infernape and Garchomp,) and some of them were weird (Dialga and Palkia.) Then I saw Torterra. I instantly said to myself, I gotta have it. So about a week later, I walked out of my local Game Stop with Pokemon Diamond in hand. And the rest, as they say, is history.
My experience with Torterra was about as good as I expected it to be. A bulky Grass/Ground type was (and still is) a unique type and in-game, it proved to be one of the best types I could have asked for. Torterra took down Roark, Maylene, Crasher Wake, Fantina, Byron and Volkner. That's six of the eight Gym Leaders on its own. The Elite Four didn't fair much better. Bertha, Flint and Lucian all fell to my Torterra's wrath. Even Cynthia broke before the might of my (stupidly overleveled, I'll admit) Torterra. I got so into this Pokemon that I neglected to train the rest of my team, a mistake I hadn't made since my Totodile in Pokemon Silver. To this day, some of my best memories from Generation IV are of my Torterra. I've trained a Torterra in Pokemon Diamond, Pokemon Platinum, Pokemon Soulsilver, Pokemon Black, and Pokemon White 2. That's a lot of Torterra. And yes, they're always incredibly fun to use.
Competitively, Torterra hasn't fared quite as well. Currently stuck in NU due to its massive weakness to Ice type moves (why use a Torterra when you have a plethora of sexy dragons to choose from? And they all have the same weakness to Ice) and low speed. However, Torterra does have a few things going for it. Not only does it resist Stealth Rock, it can set it up as well. It's typing grants it an immunity to Electric type moves, Rock Polish fixes its speed issues, and it is a serious powerhouse with a solid 109 Atk stat and access to moves such as Wood Hammer and Earthquake. Torterra can be used successfully in higher tiers. Especially if you consider its synergy with Sandstorm teams.