I always just called it the crust, but there are some interesting discussions on the links below! You're not the only one who's been wondering about this!

Though they both would be exactly the same if the whole loaf were cut at once; the heel, being the last slice, is usually a bit dried up on the inside and the crust is a bit leathery. So, I think you need to use an expression that indicates that this is actually something nice. Decent bread is still something relatively novel in Britain (particularly north of Watford), which is probably why we don't have an equivalent word.

Perhaps the comment in brackets above is just because I'm a Londoner!

French term (edited): first slice / heel

The Wordreference crowd thinks that "heel" works for "entame," though I usually only use it for the slice of bread that is left at the end.

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Note added at 15 mins (2011-07-19 15:11:05 GMT)
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Oops, I meant to enter my ENGLISH TERM as "first slice / heel." :o

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Note added at 16 mins (2011-07-19 15:12:20 GMT)
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And again, Answers entered before mine were not visible to me. :-|

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Note added at 3 hrs (2011-07-19 18:27:00 GMT)
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The more I google it, the more convinced I am that "heel" refers to the last portion of bread, cheese, salami, etc. So I think first slice is still the only choice. :-/

"Entamer" means to start. The "entame" is the first slice of a crust. I'd be tempted to say "first end slice" but that sounds odd. Maybe even "first slice with the crust on it" may do the trick although it is horribly long. Sometimes there is no other natural way round it!

Well, for a loaf of bread, we usually say 'crust', and for other things 'heel' (though that is used for bread too). Cf. the FR word 'talon' for the same thing (but usually referring to the last bit left, rather than the first bit...)

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Note added at 18 heures (2011-07-20 09:05:57 GMT)
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I think 'heel' is a somewhat old-fashioned term now, but certainly people of my parents' generation did indeed use it for the first slice (specifically, of bread, which is the commonest case where it crops up) — though I suspect this was indeed a mis-use, in that no distinction was being made between the first or last slice.

But I can only agree with other answerers and commenters: in the general case, 'first slice / piece' is about the only viable term in EN that suits most cases.

I would always use "crust" for bread but where this would not apply e.g. cake or pâté in a loaf shape then I would just use "end" or "end bit". Things my Mum used to say and I said in my turn:

I suppose you want the crust again?
Who wants the end (bit/slice) of cake?
If you like the meat jelly, you can have the end.

In spoken, informal English I think we would say this rather than "first slice", "last slice" etc. Of course, end can refer to the last slice, too but you've got the context in front of you!

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Note added at 4 days (2011-07-23 15:55:51 GMT)
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You aren't alone, Andrew! I never heard the term "heel" used in this context during the 38 years I was in the UK.