OK....pros and cons...pros and......cons:
Crobat-Cheap 4X Poison attack, when Unown G leaves, Crobat will be all over the place. Wrecks Donphan Prime, and is a decent Sniper. Bad weakness is probably the worst thing.
Kingdra-Got my vote, Stackable damage counters wherever is great short of Nidoqueen RR, but that 10 Damage could just be enough to KO something with 60 Damage in 1 Water Energy. Also bad weakness.
Lanturn-Donphan Wrecks this, provided it gets the first hit, which it will 95% of the time, if not more. The attack is powerful, and "dual type" can hit for weakness on most of the competitive decks, short of Gengar. Heavier Retreat.
Steelix-Definately has the best top and bottom stats, with special metals and an excellent pokebody, 140HP will go a long way on a stage 1. Fire will probably get a bit more play when Ninetales is the next Claydol, Infernape and Blaziken will be around. Handy Psychic Resistance. Does retreat cost really matter? Even if it was 2, you probably wouldn't want to retreat this monster anyway.
Tyranitar-Bad Weakness, good resistance, Hefty retreat. Great Spreader, would work fine with Donphan Prime's Pokebody, I can see a batch of these decks around..D Prime/Ttar, throw in Luxray GL and you got a team (and Spiritomb...who could even attack! Le gasp!)
Ursaring-The best thing about Ursaring is DCE Compatible, but with 110HP, it gets wrecked by Donphan, even Kingdra can 2KO it before it pulls off an attack. Big retreat, no resistance, expensive (= no T1 rare candy tricks
....)
All in all, I'd say:
1. Kingdra (Pokemon Power = Crobat G.)
2. Crobat (Cheap 4X poison is great)
3. Steelix (Fully load this one on the bench, and he'll haul through)
4. Tyranitar (Stage 2 with a good, but possibly hurtful spinning tail)
5. Lanturn (Wrecked too easily, and a little slow assuming you don't have Feraligatr Prime on T2)
6. Ursaring (Too expensive attacks vs Donphan)