Personally, I feel any of those would be worth a Gardevoir. While Serperior, Samurott, and Typhlosion might not get you as much as they could have last gen because of how readily they can be found with Island Scan, I think they'd still fetch well on the GTS due to how Island Scan is convoluted enough that most won't attempt it. And they'd definitely be able to get you a Gardevoir; it can be found quite early in the game in both Gen 6 games, and it reaches its final evolution by level 30, which, considering what levels other 3-stage Pokemon reach their final form at, is pretty low. However, Gardevoir is still pretty rare, considering that since Ralts is found early in the Gen 6 games, and as a result most people might not take the time to train it up to level 30, and the ones that do will want to keep it, so you may want to lowball it and ask for a Kirlia, or even a Ralts. And as for which one would be worth the Gardevoir, it depends. Like Fizz said, Contrary Serperior is very valuable, and will without a doubt get you a Gardevoir. However, if you're trading over the GTS, or don't have Contrary Serperior, then I'd go with Typhlosion. Like I mentioned earlier, all three of those starters can be found through Island Scan. However, Samurott and Serperior, when using Island Scan, are found straight in their fully evolved forms. Typhlosion, however, will be found as a Cyndaquil through this method. So if someone wanted a Samurott or a Serperior, they could just Island Scan for one and get it instantly. With Typhlosion, though, they'd also have the added burden of training it up to level 36. Many people may not have the time or patience for that, so they would trade for it, which is where you come in.