Traveller. I've played tonnes of roleplaying games, but I always return to Traveller. I still have my collection of original little black books.
I'm very much a science-fiction nut, so there's that reason. Traveller very much lives in the realm between hard science-fiction and space opera.
I also have a strong preference for rules light games versus the rules heavy. Rules light tends to allow for better storytelling. Rules heavy games tend to get in the way of storytelling. In my experience. Traveller is certainly not rules light in the vein of Savage Worlds, Risus or Big Eyes, Small Mouth, but neither is it rules heavy in the vein of Dungeons & Dragons, Hero System or GURPS. It lies in the comfortable in-between.
EVE Online is the closest we have to the Traveller universe online. It is the closest we have to Traveller in tone and substance. There are more similarities to Traveller in EVE, than not. I would be surprised if Traveller was not a major inspiration in the design of EVE Online.
I initially found the crossover from table-top roleplaying to online roleplaying easy. I first made the transition with Ultima Online. The Napa Valley shard, roleplaying with a group called The Sinful. We had a set of guidelines on who we would prey upon and how we would prey upon them. We roleplayed evil. We were often referred to as PKers (player-killers), the early definition for what's now known as griefers and gankers. We roleplayed it, and always offered our prey an out, if they'd roleplay along with us. If they didn't, then all their stuff was ours for the taking. We didn't engage in exploits, and we didn't seek tears and rage. We kept our play in-character. We were certainly misunderstood: that's one of the difficulties of roleplaying online, where the vast majority of players have no interest in roleplaying or no understanding of roleplaying, your actions and motivations are attributed to nothing more than greed and psychopathic behaviour. But we kept to our principles and over time garnered a fair bit of respect from the other players. Eventually we were no longer referred to as PKers, and people started to enjoy their unexpected encounters with The Sinful (if not so much their losses.) A couple of roleplaying groups even sprung up to oppose us.
From Ultima Online, I moved to Everquest. The Rallos Zek server. At the beginning of Everquest, Rallos Zek was the only PvP server. I'm a strong believer that real roleplaying cannot occur without conflict. On any other EQ server, my Ogre would only be capable of standing before his elven enemies, yelling angry slurs at them. That's not roleplaying. That's unnecessary limitation. On Rallos Zek I was able to teach them lessons via my club. I actually organized the first major battle on Rallos Zek, over a hundred combatants on both sides in East Commonlands (which crashed the zone for a couple hours.) And a group of about forty of us created a series of interlocking stories that we'd play out online, and then write about. Creating our own plot-driven version of Everquest.
Roleplaying has always been difficult in an online massively multiplayer environment. Beyond Ultima Online, MMOs became increasingly geared towards number crunching and loot. Cooperation became tantamount over all other concerns. You might be bitter enemies with an elf one day, but because you needed their skills on a quest or raid the next day, you found a way to contrive roleplay reasons to accept them into the group. Roleplay became less about creating a character and staying in character, and more about figuring out how to rationalize why your character was doing things so obviously out-of-character.
My group in Everquest slowly drifted towards this contrivance. As members of the group levelled, quests and dungeons and raids became the all-important consideration for character advancement. I disagreed with this surrender to the game's design over actual roleplaying. I didn't blame those players for their change in priority. The game itself demanded that change if you wanted to be successful at it.
Rationalizing what was convenient for the player rather than what was appropriate for the character. That's what MMO roleplaying had become, so I stepped completely away from it. The blame for this could be placed squarely on MMO design. There were backstories to the games, for sure, but the game mechanics didn't lend themselves to roleplaying. The games pushed acquisition as the primary means of character development, which wasn't all that compatible with creating and maintaining a character.
So for the next few years, I hopped around between MMOs. Anarchy Online. Asheron's Call. Dransik (now knowns as Ashen Empires.) I looked forward to other games that petered and failed on release, such as Shadowbane and Matrix Online.
I joined World of Warcraft at the start of the Burning Crusade expansion. It seemed that maybe a return to online roleplaying might be possible. Two sides locked away from each other. A reasonably deep and expansive backstory. Open-world PvP (again, roleplay without conflict is barely roleplay.) But no, WoW was little different than any other MMO, maybe even worse. A treadmill of acquiring items upon items upon items. Roleplay took a backseat to the hamster wheel of loot acquisition.
I don't mean to paint all roleplayers with the brush of contrivance, even if the majority of my MMO roleplaying experiences ran up against such artifice time and again. There are certainly online roleplayers who can stay in character, who will give up gain for the player because it doesn't fit with the concept and personality of the character. I respect those people. Because that sort of play is difficult, especially when games reward nothing but climbing the loot ladder. Their perseverance in the face of so many obstacles, it is admirable.
EVE Online, like Ultima Online, certainly has the most potential for online roleplaying. Sandbox games do. Players tend to create the content, they tend to create the engaging stories. Conflict is a cornerstone of EVE. I've encountered more roleplayers in EVE than any other game I've played. Most certainly. If I'd been introduced to EVE Online back in 2004, I might have began as a roleplayer, and might still be a roleplayer today. There are certainly the die-hard roleplayers who rarely break character, Roc Wieler for instance. There are great roleplaying groups, such as Ushra'Khan. But I still see many instances of contrivance, even among groups like Ushra'Khan, and because of it I remain disenfranchised from online roleplaying.
Take my character in EVE, Poetic. Born Gallente. Joined the Minmatar militia. Moved to the Amarr militia. From a character-based perspective alone, that would have never happened. Minmatar to Amarr? For a liberal Gallentean? Never. I moved to Fweddit because I knew I'd have fun. Sure, I could have concocted some dumb story about defecting and all that jazz, but it would have been pure contrivance. Stepping out of character to satisfy the needs of the player. Roleplaying online, it's become too much of a bother for me now. I'd rather just play to satisfy my own needs. I still respect all the roleplayers out there, and I'll engage in some comedy roleplaying from time to time, but there are too many hurdles to cross in online gaming to roleplay properly or effectively. So I opt not too. I'll keep my roleplaying to the table-top.