I bottom fill my doggy and berserker. I don't really like it, very inconvenient when you're out and about. But I put up with it because they're both a lovely vape.
I really like the twisty top fill on the ehpro true, but that implementation is seriously flawed. The port holes are way too small, it's hard not to get juice all round the top, and the rubber seal (on mine at least) seems to be poor quality and has started to come away. Also, the white bits you can see when it's closed make an otherwise decent tank look really shit. Also, if you're a bit dozy, it's easy to forget to close them and it leaks all over the place.
Hastur mini lipped twist style - I really like this because it's so easy to use and there is no leaking. But I use mine a lot and I'm sure they're starting to get looser. I'm never fully confident that it won't come off in my pocket or bag. Never has done, but..... the siren (which has a similar fill cap) feels much more sturdy. But sadly it doesn't vape as nice as the Hastur.
The savour is one of my favourites. A standard screw, but has none of the problems of the beserker when top filling. I don't know why. And the fill ports are very generous. Ditto for the Wake RTA.
Hate the OBS style side filling. Really awkward, prone to mess, and makes the tank design unnecessarily complicated / hard to clean.
I second the thoughts on most slide top fills, but I find the slide style on the Ares to be ok.