Yes, the Tempus' power word domain spells will stink in HoF mode. But the high level Monster Summoning spells that are the alternate choice to the power word spells are VERY useful.
The choice of "best" cleric is, indeed, very dependent on the role you envision. it can also be dependent on whether you intend of playing HoF mode.
I agree that ML's of Lathander are wonderful offensive spellcasting clerics. However, their fire-based domain spells will not be nearly as potent and useful in HoF mode.
Stunning, "charming", and monster summoning spells are more useful offensive spells in HoF mode than many damaging spells.
Ilmaterians are surprisingly potent in HoF mode.
3 Magic Circle Against Evil
4 Emotion: Hope
5 Holy Power
6 Stoneskin
7 Holy Word
8 Holy Aura
9 Summon Monster IX (Horrid Wilting is a good alternative)
Are all solid HoF choices in HoF mode.
Holy Word and Symbol of Hopelessness are excellent HoF mode spells. Watch out with Holy Word however, since it stuns all non-good characters inside its range. This will (or may) include any non-good party members. (I've been playing 100% good parties of late.) But while Holy Word has this downside, it is also very, very useful in a tight situation because it has a casting time of 1. It also centers on the caster. The combo of HW and SoH is great. Cast HW to stun enemies in close. Cast SoH to stun enemies at range.
Silverstars can be quite nice if you like elementals. I don't really like elemental summoning because I prefer to be able to control most of my summoned monsters. However, sometimes against some of the mobs in HoF, you cannot afford to be picky.
All in all, the "best" cleric is very dependent on your cleric "playing style", normal vs. HoF mode, preference of good vs evil. Lathander and Talos both make good offensive spellcasters. Tempus, Helm, and Talos are good in melee. Banites are good "enchanters". The other clerics are always pretty good all around.
Some clerics' special abilities are more useful than others. I played an Ilmaterian all the way thru normal and HoF and never once used his special ability (never really remembered to do so). The 2 feat points in Axe are great for a Tempus cleric, cuz they are useful and you don't have to remember to use them. The Lathanderian healing touch ability is quite useful. The Banite's "Tyrant's Dictum" ability is useful since so many of their Domain spells are require Will saves. I find Oghma and Mask special abilities to be rather weak.
The Helm abilities are useful, if you remember to use them.