I tend not to think of Sun Titan as a tournament card, which is totally unfair of me. It's not as high-profile and enduring in Legacy as Primeval Titan or Grave Titan, but Sun Titan has seen its fair share of tournament play. Its versatility probably has it edging out Inferno Titan in that area. Getting a permanent back, often multiple times, is a powerful effect. While I won't try to capture the full scope of Sun Titan's tournament appearances, I do want to highlight some of them, particularly ones that date back to when I was less acutely aware of the card's existence.
In Legacy, there were three major tournament archetypes that adopted Sun Titan during what I've identified as my fourth lull in Magic-playing activity. I was going to simply list them, but of course not everyone is familiar with these archetypes, so some introductions are in order. By the way, none of these were what I thought of, at the time, as "Sun Titan decks." They were very much doing their own thing, but sometimes Sun Titan was a good fit.
Maverick
In the wake of the Legacy banning of Survival of the Fittest, green-based toolbox decks went dormant, but it wasn't long before the printing of Green Sun's Zenith in Mirrodin Besieged caused a resurgence of this general concept. There have been other GSZ (Green Sun's Zenith) decks, but the "Maverick" archetype was the one that essentially imitated the white/green Survival decks that existed in Legacy before the rise of Vengevine and the banning of Survival of the Fittest.
Maverick is distinguished by the presence of 4x copies of GSZ alongside Knight of the Reliquary and Stoneforge Mystic. There's nothing intrinsic to those three cards that makes them seem like they belong together and it was really the way that they fit together into a deck with disruptive creatures that caused this to even work. Supposedly, the very earliest versions of the deck were based around Aether Vial before GSZ came out, and the name "Maverick" was based on the idea that they deck was "off the radar."
Sun Titan was certainly not in early versions of Maverick, but it seems that at some point while I was in college, people started throwing it into Maverick lists to top out the mana curve. It makes sense. Maverick could act quickly, but the deck was always more about utility and disruption than about trying to race opponents. It won through attrition. Typically, this involved establishing a lead early on in a game. Sun Titan could close the deal. Why Sun Titan and not some other big dummy? Because almost every other permanent in the deck aside from Batterskull could be brought back by Sun Titan. If an opponent was able to slow Maverick down by blowing up Umezawa's Jitte or Knight of the Reliquary, Sun Titan could bring those cards right back. A popular target would have been Eternal Witness, which could itself grab any card out of the graveyard, with no limitations. Six mana was a lot, but between GSZ for Dryad Arbor, Noble Hierarch, and Knight of the Reliquary for Gaea's Cradle, this was a deck that could reasonably get there.
Dredge/Ichorid
Here's an archetype that's well-known outside of Legacy, and I think so well-known that everyone at the CPA recognizes it. I talked about Legacy Ichorid decks, later known as "Dredge", in a few of these Magic Memories threads. To recapitulate, this was an archetype that used Golgari Grave-Troll, Stinkweed Imp, Golgari Thug, and sometimes other cards with the "dredge" mechanic, to replace drawing cards with self-mill. They ran ample discard effects, including effects that could draw multiple cards, but those draws would be replaced with increasing amounts of self-mill. This could be accomplished very quickly, with an outsized graveyard being built up by turn 2. Ordinarily, discarding one's hand and milling oneself would be bad, but these decks then relied on an engine based around five other cards: Narcomoeba, Ichorid, Bridge from Below, Cabal Therapy, and Dread Return. Narcomoeba went onto the battlefield when milled, and could then be sacrificed to Cabal Therapy or Dread Return. Ichorid could enter the battlefield from the graveyard and would automatically die at the end of the turn. Having Narcomoeba and Ichorid die would generate zombie tokens with Bridge from Below. Cabal Therapy was a sacrifice outlet and could strip disruption out of the opponent's hand. Dread Return was another sacrifice outlet and could be used to put a big creature onto the battlefield.
That's a quick summary, but it might be hard to appreciate how this deck works if you've never seen it in action. Dredge decks routinely get two or three copies of Bridge from Below into the graveyard, two or three Narcomoebas onto the battlefield, and two or three copies of Ichorid onto the battlefield, all by turn 2. This means that an opponent is staring down 8–18 zombie tokens alongside whatever big creature Dread Return was used to cheat out, and that Cabal Therapy has been used to get rid of whatever cards in the opponent's hand constitute the biggest obstacle to the zombie army swinging in for the kill.
Dread Return targets in Dredge decks have always been a bit different from the typical targets a Reanimator deck would use. Sure, a Grave Titan could work here, but when you're already making a zombie army, it's better to use Dread Return on a creature that does something else instead. A go-to target in the early years was Angel of Despair, which could blow up a pesky permanent and also swing in for 5 damage. Flame-Kin Zealot was a greedier option, as it could boost the zombies and give them haste, aiming for a faster kill with fewer zombies, although attacking a turn earlier generally meant forfeiting one's copies of Bridge from Below in the graveyard. Sun Titan emerged as a kind of backup target for Dread Return. While there weren't a lot of gamebreaking permanents to bring back at CMC 3 or less, Sun Titan could do a neat trick of bringing back Lion's Eye Diamond, which could get valuable cards out of your hand and into your graveyard, where you needed them (yes, Dredge is a weird deck like that). The mana could then be used with Cephalid Coliseum and Deep Analysis to generate more card-drawing, which meant more dredging, which meant more zombies. Using Grave Titan and making four more zombies would be a mistake when you could use Sun Titan and make ten more zombies. In a pinch, Sun Titan could also bring back Putrid Imp to get another option for a discard outlet.
Nic Fit
Maverick and Dredge made good use of Sun Titan, but it was never really popular in those archetypes and I think they consistently dropped the card many years ago in favor of other options. Some rogue decks have used Sun Titan as well. But the major tournament archetype in Legacy that is most closely associated with Sun Titan, the deck that used Sun Titan most frequently and for the longest amount of time, was definitely Nic Fit. I don't think that Sun Titan has turned up lately, but Nic Fit is a silly deck and you can run pretty much whatever you want in it, so I wouldn't rule it out. If you haven't seen a Nic Fit deck in action, you're missing out. I suspect that if the members of the CPA had our own Legacy team, Nic Fit would have been a popular deck for us, because when playing it you get to do oddball stuff and make it work.
Nic Fit operates by sacrificing Veteran Explorer to Cabal Therapy and using the mana to cast Green Sun's Zenith. I already mentioned Maverick, and it might seem strange to associate Sun Titan with GSZ decks when GSZ can't fetch Sun Titan, but really, these archetypes don't have much in common. Maverick is a toolbox deck powered by disruptive and utility creatures. It can ramp mana a little bit, but mostly just attempts to use early disruption to take over the board, along the lines of an old Survival deck. To Maverick, GSZ is an engine. To Nic Fit, GSZ is a bomb. Cabal Therapy on Veteran Explorer lets you mess with your opponent's hand while ramping both sides of the field, as long as both players have basic lands to fetch. Despite being a multicolored deck, Nic Fit tends to run far more basics than most Legacy decks, which have always been light on basics. So opponents might get ramped a bit and be able to curve out, but the Nic Fit deck is deliberately built around jumping over the mana curve. So opponents with their cute little efficient creatures get overwhelmed by big, powerful cards.
Some of the cards historically used in Nic Fit decks have never really had a presence in any other Legacy archetype, because Nic Fit is the only deck that can get away with running those cards. Sun Titan isn't one of those, but it's a sensible choice here. A six-drop is no problem for Nic Fit and recursion is excellent in a deck that frequently sacrifices its own creatures as well as playing board wipes like Pernicious Deed. If I remember correctly, every Nic Fit list I've ever seen that ran Sun Titan also included Eternal Witness, so Sun Titan for Eternal Witness for something else was an option. Recurring Veteran Explorer itself can be nice under some circumstances.
