(Okay, wow, I'm not sure why I went on such a tangent but yeah I guess I kind of over-explained two cards that everyone knows hahaha. Still, my opinions here stand.)
There are two Pokemon EX I think work as very strong surprise cards. Incidentally, both of these mainly work in decks with grass energy, and I think Virizion EX alone makes grass a rather popular type. The first card would be Dragonite EX from the recent Furious Fists set. Just looking at Dragonite, he's obviously designed for surprise. His ability lets you move as many basic energy to him from your pokemon as you like, and then switch him into the active position. Basically you can be in a situation where you have plenty energy available but don't quite have the attackers to finish off a pokemon. Drop the Dragonite, and boom it's up there swinging for 80, 120 with a discard, and your opponent may be surprised and put at a disadvantage, depending on the timing. Dragonite's main flaw, really, is his high retreat cost. If you play Dragonite without activating his ability for some reason, he is slow to charge up and just as hard to switch to the bench if you happen to start with him.
Now the other card I enjoy is Shaymin EX. For any who don't know or remember, Shaymin has a rather disappointing 110 HP despite being a Pokemon EX. That being said, I think it is meant to work as a late game surprise card and a finisher of sorts so the HP isn't extremely bothersome. Now this means starting the game with Shaymin EX could be problematic, but its first attack Synthesis costs 1 grass energy and lets you search for another grass energy and attach it to any of your pokemon. This combined with a retreat cost of 1 makes Shaymin much better to start with than, say, Dragonite, despite its poor bulk. And now the real appeal to Shaymin is its Revenge Blast attack, which does 30 damage +30 more damage for every prize card taken by your opponent. Certainly a terrifying card late game, hitting for 180 when your opponent has only one prize left. I'm very attached to this card personally after a game at my first Regionals tournament. My opponent, playing CMT, had taken 5 prizes, I still had all 6 left. I was in trouble. And then wouldn't you know it, I get my Shaymin out (or maybe it was already out, I don't quite remember how I so easily got it going without it getting KO'd haha) and I go for a solid sweep and take all 6 of my prizes, using Eviolite to survive the one or two attacks my opponent could get going before I KO'd all his pokemon. Greatest comeback I've ever experienced. Now I know not very many people agree with me on Shaymin's playability, but I strongly urge you to look at the recent Worlds runner-up's decklist and tell me what you see there.
Shaymin wins games that should be lost.
