Would you say that FFIV had the "best" four fiends, when compared to the other FFs with Elemental Fiends?
Well, I guess it would be unfair to compare FFIV's fiends with the others in terms of depth and personality, so how about looks wise (or battle wise if you're more of a stat person)?
Scarmiglione: Compared to Lich, Scarmiglione was definitely more exciting looking, since Lich was a rather generic looking undead lord thing while Scarmiglione looked like a really creepy zombie with worms growing out of him. That Dragon guy guarding the Earth Crystal in FFV was definitely cooler looking than Scarmiglione, though being a dragon would be more fitting for a Wind Fiend. Arihman in FF3, being a flying eyeball isn't really fitting for an earth fiend, and between the two I'd say Scarmiglione wins when it comes to creepiness.
Best: Scarmiglione
Cagnazzo: Since I find Cagnazzo rather harmless looking (with no crocodile snout or spikes on his shell/tail), I find Kraken to be more ferocious looking than him, though I guess they both look fitting enough to be Water Fiends. Echidna doesn't really look that dangerous, or much like a water fiend, so Karaken wins there. That Mammoth Thing from FFV was also more ferocious looking, though less fitting to be a water fiend.
Best: Kraken
Barbarrica: I've always had something against having human looking fiends (since they aren't as exciting as having monster fiends). Tiamat, being a dragon, is more fitting for a wind fiend even if it doesn't have wings. Cerberus looks more ferocious looking than Barbarrica, but doesn't really look like it'd be a wind fiend. Wing Raptor beats Barbarrica, both in ferociousness and in fittingness to be a wind fiend.
Best: Wing Raptor
Rubicant: My "human fiend" rule applies here as well, but I guess he does look like he'd be a fire fiend because of his demonic red face. Compared to Marilith, I'd say they're pretty much even since they're both humans yet they look fire-fiendish enough. The Two Headed Dragon (being a dragon) definitely beats Rubicant in both fittingness and ferociousity. Liquidflame, being composed of nothing but flame, is definitely more fitting of a fire fiend, though it kind of doesn't look ferocious because it has no eyes or any absolute form for that matter.
Best: Liquidflame.
Overall, I'd say that FFIV's fiends had each of the fiends represent their elements well, though I'd say that FFI did an overall better job at making fiends that represent each element. I know that it was mostly FFV fiends that won, but the two FFV fiends that weren't fitting at all for their element cancelled it out.