For my dad. His upgrade choices are:

Nokia Lumia 800
Sony Ericsson Xperia V
Samsung Galaxy Ace 2
HTC Desire
Motorola RAZR i (I suspect - he's written "VISAi")
Blackberry 9790

No clue about any of them except the Blackberry and I know he wouldn't use it because his fingers are like mine - short and stubby. He uses his phones for calls and texts only. He will probably use it for music.

HTC Desire? That's a two and a half year old design.
I have one and in general I really like it. There are just a couple os small issues.
1) Internal storage. There isn't much and not all apps can be moved to the SD card. That means it gets full and as all apps have to be installed to the phone before they can be moved to the SD card you could get to the point where you can't add new apps withoit removing old ones. (I'm at that point now.)
2) After 2 1/2 years my menu button is intermittantly iffy. Better get a phone where those buttons are part of the touch screen.
On the plus side I really like HTC's Sense interface. Having played with a friends Samsung Android phone, I can say the HTC feels better to use. The Desire screen is good too. Bright with a sharp image.
Personally I'd steer clear of Blackberry. The apps just aren't available for it compared to Android/iOS.
I've no idea about the others.

EDIT

Do you mean the Desire C rather than the original Desire?
That has no buttons, and better storage, but has a much poorer screen resolution.
The Desire X seems to be the best replacement for the original. More internal storage, no buttons and a good screen.
The Desire V seems almost as good, but has a bit less RAM.

Sorry. Desire C. Though my dad's heard negative reviews from work colleagues so he's not keen on it.

The Sony Xperia U is a very capable & highly popular budget handset. I believe it currently runs Gingerbread but there's an update to ICS in the works.

Yeah, the resolution is pants, it's like an old Wildfire. The Desire X looks good though if he can get that.

I have this, but basically had to buy it off eBay. Take a look around though if it would suit him;

http://en.wikipedia.org/wiki/Sony_Ericsson_Xperia_pro

Responsive, runs ICS, might even get Jelly Bean. It actually is the best of both worlds with that keyboard.

These are free upgrades from his network and he doesn't want to pay anymore.

Ah, that's what I get for skim-reading

NOT the Nokia Lumia 800.

My mate's dad has the Galaxy Ace, and seems very comfortable with it - don't know how much it differs from version 2 though.

I've got the Lumia 800 as a work phone. It's very good if you discount the fact that it won't get Windows Phone 8

I would say the Nokia Lumia or the Motorola Razr i.

That said, the Lumia is about to be replaced, so I would hold off a couple of weeks, the newer Windows Phone 8 version of the 800 series is very nice.

The Motorola RAZR i uses an Intel processor, but is a very capable and mid-to-high-end phone. It comes with Android ICS.