Oookay, let's see if I can remember what I thought about my team at the start. Bear in mind I like to overlevel (though because of those rebattlable breeders, that's really easy)
Riolu: I'm saying high. Caught him with a Jolly nature, so the natural choice was a physical attacker. Took a little while to level him up to get Force Palm, but he levels up really quickly, faster than Azurill. Once he got that, I used him pretty much any time nothing was super effective. The thing that really sold it for me was the bug gym. I had no fire, flying or rock, and he was already a Lucario, so I just sat there and took all the hits easily, and Force Palm did decent damage. Also taught him Ice Punch, and spent fun times punching Drayden and Iris' dragons in the face. The only trouble with him is before he gets force palm, and the amount of Pokemon with a super-effective move which instantly KOs him, though the steel type means so many hits do no damage at all. He does great against Plasma, and Close Combat OHKOd Black Kyurem in a very anticlimatic way. EDIT: Paralyze on Force Palm was useful, especially for catching Pokemon.
Azurill: Probably mid. Got one with Huge Power, and it really does have a lot of power. It was my choice against fighting types, since it was fairly bulky, and it was a good hitter when I could only manage neutral. However, it sucked before being an Azumarill, and it really lacks coverage. I didn't teach it ice punch in the end, especially seeing as it ended up with 3Hms on it.
Mareep: High. Even in early game it kills stuff pretty quickly with thundershock. Unfortunately it has a lull between from after Elesa to before Skyla, where it learns Discharge, where it fails to OHKO. However, it's bulky enough to take a few extra hits. Also once it gets a bit higher level, it picks up some nice coverage moves. EDIT: Static activated often, and was in fact useful.
Sandile: High. Always gets the job done. Dig is annoying to use though, and then it gets EQ, which is annoying in doubles and triples. However, with Intimidate it can take a few physical hits, and Dark type is a very good type to have. Does good vs Plasma too.
Volcarona: Low for the main game, but including post E4 it's mid. Hardly ever used it, since it comes way too late for the bug gym. When I tried to use its awesome special power on fighting types, they OHKOd with Rock Slide. Has no decent attacks for ages, they either have low power or miss. You'd think it would be low, and for the main game it is. But it can do some damage to the E4 (since you have Flamethrower), and if you need to take a special attack, it's amazing. Post game is where it really shines. If something tries to use a special attack, Quiver Dance for as long as you like, then Flamethrower the rest of the team to oblivion (for example, Alder). It still lacks coverage, though, but who needs it at this point?
One extra point in its favour: for a team with no HM slaves, something with Fly is hard to find. So Volcarona made itself invaluable in that way at least!
This is what I can remember. If anyone wants to discuss my points I may remember something and revise my opinion a little. I hope my post was informative.