I can't really choose one:
Death Note- Dub. If it wasn't for this, I wouldn't have gotten back into anime.
Higurashi no Naku Koro ni- Sub. The dub is terrible, but I found it fun to listen to after finishing the sub. This was the first anime I saw subbed.
Welcome to the NHK- Dub. This anime is just fantastic in every sense.
Hellsing Ultimate- Dub. Mostly because I love the accents, and it's weird to hear them speak Japanese in London. Truthfully, I couldn't get into the original series, but the OVA pulled me in with a better story and lots and lots of blood.
Black Lagoon- Dub. It just fits the anime better. It like something Robert Rodriguez would film.
Fooly Cooly- Dub. This was my favorite anime when I was younger, and still is, and I love the soundtrack.
Full Metal Alchemist- Dub- I haven't seen this anime in so long, but I remember it being one of my favorite animes.
Trigun- Dub- I recently revisited this, and was reminded on how awesome it was and still is.
Umineko no Naku Koro ni- Not as good as Higurashi, but that doesn't mean it sucks.
High School of the Dead- It just came out, but I'm loving every minute of it. I also started reading the manga, and it's great.
Clanned- Just started watching this, it's hillarious, heartwarming, made make me cry manly tears of joy.
Cowboy Bebop- Dub- I haven't seen this one for a long time, I should watch it again sometime.
School Days- The polar opposite of Clanned. I should hate this anime. Despicable cast of characters, rage inducing moments, the depressing story, and that ending...that ending... made me realize how brilliant this anime truly is. It's pretty good, for an anime based on a hentei game.
Code Geass- Death Note with giant robots, but better.
Elfen Lied- Probably the first time I actually cried during an anime.
Hope I didn't miss anything.