Skies of Arcadia: Best game ever made in my mind it brings tears to my eye when I see it...
Sonic the Hedgehog: No need to explain
Sonic and Knuckles: Just plain great!
Vitrua Fighter 5: Just a plain great fighter
Sonic Adventure: It was my whole dreamcast
Sonic Adventure 2: Last time sonic has made me smiled on a console Excluding Brawl
Vectorman 1: ....I wanted a third one....

Cant think my opinion keeps changing as sega games I loved keep poping into my head. Will be back with them as soon as I can come up with a list and stop remembering another 3 great games.

Sonic the Hedgehog and the original Nights on saturn. Also skies of arcadia for the dreamcast

Yakuza series.
Valkyria Chronicles.

Well if it's sega developed only.. then that's about it :p

For me that would be

1.Revenge of Shinobi
2.Shenmue 2
3.Shenmue 1
4.Skies of Arcadia.
5.Afterburner.
6.Sonic the Hedgehog 2.
7.Phantasy Star 4.
8.Legend of Thor
9.Virtua Tennis
10.Panzeer Dragoon Axel/RPG

Sonic Heroes (I really like the intro song).
Sonic adventure.
Sonic adventure 2.

Yay! Tis a glorious day indeed!

Anywho, I'd say I'm more of a Sonic fan than a Sega one. Still, it kinda goes with the territory.

In no particular order, here's some Sega games that I've fallen in love with.

Sonic & Knuckles (the greatest game of all time )
Sonic the Hedgehog 3
Sonic the Hedgehog 2
Sonic the Hedgehog
Sonic Spinball
Sonic Rush/Adventure
Sonic Advance 1-3 (Seeing a pattern? )
Dr. Robotnik's Mean Bean Machine
Sonic & the Secret Rings (Not perfect, but it's a lot of fun)

It's true, my Sega diet is horribly one sided... ah well.

Sonic Adventure 2: Battle for the win.

Sonic the Hedgehog 1/2/3/Knuckles (Mega Drive)
Streets of Rage 2/3 (Mega Drive)
Ristar (Mega Drive)
F-Zero GX (GC)
Sonic and the Secret Rings (Wii)
Sonic Rush Adventure (DS)

I no particular order...

I forgot about Sonic CD as well.