Our first impressions of Admiral Croft are of an amiable but somewhat vague, unperceptive man, rather overshadowed by his shrewd, evidently intelligent wife. What is his 'secret ingredient', that has made such a woman so strikingly happy after fifteen years with him? On closer inspection I believe he has many good qualities which combine to make him excellent husband-material. (What follows is in no particular order.)

Successful in his career. The rank of Admiral simply reflects his seniority in the Navy since he obtained the rank of Post-Captain. He may simply have inherited money, but the way he speaks of the Navy, and his present affluence (compare Kellynch with the inconveniences of his early lodgings at North Yarmouth and Deal), suggest that he has also been successful as a Navy officer. You don't manage a warship or a squadron, or capture enemy shipping, simply by being courageous but vague. It also requires intelligence, so I think he is a lot sharper than he appears.(8,18)

Good humoured. This is mentioned several times, Even the touchy, suspicious Sir Walter is disarmed by the Admiral's 'hearty good humour'. Unsurprisingly he has a great many friends; the streets of Bath are full of them.(5,18)

The Admiral is also amusing, with his vivid, vigorous way of expressing himself. Sophie with her blister is "tied by the leg"; Wentworth's letter telling of Benwick's engagement contains "not an oath or a murmur from beginning to end".(18) Definitely a man who would keep a smile on your face.

Frankness and 'simplicity of character'. (13,18)
He is an 'open' character. It is impossible to imagine the Admiral plotting behind anyone's back or secretly nursing resentment. What you see is what you get.
He doesn't 'edit' himself much, and says what he thinks, for example when remarking to Anne about the mirrors, the laundry door and the breakfast room chimney that smokes at Kellynch.

Considerate & sympathetic Anne finds the Admiral's 'goodness of heart and simplicity of character' irresistible.(13) He is quick to make amend if he thinks he may have offended or distressed someone, and considers how others may feel. For example when Anne visits the Crofts at Kellynch he says "this must be very bad for you... finding us here..", and invites her to look all over the house and 'slip in from the shrubbery' whenever she likes.(13)
When he sees Sir Archibald Drew and his grandson he thinks of the difficulty the grandson will have getting promotion, and calls the grandfather 'Poor old Sir Archibald!'(18)

Tolerant The Admiral does not try to force the world into conforming to his own opinions, and is very relaxed about others seeing things differently or having different tastes :-
' One man's ways may be as good as another's, but we all like our own best;'(13)
'If the girl likes another man better, it is very fit she should have him.'(18)

He is reluctant to be too negative about people; a negative usually tempered by a positive:-
Benwick is 'too piano' for the Admiral's taste, but an 'excellent, good-hearted fellow', and an 'active and zealous officer. 18
'Sir Walter will never set the Thames on fire, but there seems no harm in him'.(5)

Even Admiral Brand and his brother, who 'got away some of his best men', are treated leniently.
Mrs Croft "cannot bear them", but the Admiral merely describes their treachery as a "pitiful trick and calls them "shabby fellows"; with many men that sort of double-crossing would result in settled enmity.(18)

Easy going, Laid-back. A couple of examples are his invitation to Anne to come to Kellynch whenever she likes (13), and his comment about Henrietta's engagement - 'If the girl likes another man better, it is very fit she should have him.'(18)
The Admiral's easy-going nature seems evident to me in almost everything he does, but I think it important enough to note it separately. Admittedly his relaxed driving style might be a bit more of a liability in the cut and thrust of modern motoring, but he would be entertaining company while he took you dawdling comfortably along the country roads, enjoying the view, not frightening the life out of you in the fast lane of the motorway.

Unpretentious:
Admiral Croft is not obsessed by his image and personal appearance as we see from his reaction to Sir Walter's large looking-glasses - 'Oh Lord! there was no getting away from oneself.'(13) (This could also be read as a metaphor for the Admiral's more general lack of narcissism and self-centredness.) Sir Walter thinks he should rearrange his hair.

The Admiral is not given to display in his choice of a carriage. The Crofts drive a gig - two wheels, one horse - minimal transport for two. (Contrast this with Sir Walter's four-horse carriage with his coat of arms on the side, and his servants wearing livery with orange facings.)

The Admiral's attitude to where he lives is interesting. Doubtless he enjoys Kellynch but he is not concerned with it's splendour - in Bath he speaks of it in exactly the same terms as the Crofts' early lodgings at North Yarmouth and Deal.18 He likes to be comfortable and 'snug', but what really matters to him is what is going on in a place, not whether it is elegant or grand.

Unpretentiousness shows in his speech too - not for him the polished platitudes of elegant conversation, and there is no hint of pomposity or self-importance about him.

Great rapport with children. The little Musgrove boys take to him immediately, and don't want him to leave. I love his line about carrying them away in his coat pocket - and so do they.(6)

Uxorious. This isn't quite the right word, as I see nothing excessive in the Admiral's affection for his wife, but togetherness is clearly very much his style. He doesn't feel right without a woman on his arm, and that woman is almost invariably Sophy. When walking up through Bath with Anne, one acquaintance assumes she is his wife, another stares to see him with anyone else. The Crofts do everything together- visiting, driving, looking at their grass and their sheep. He takes his wife to sea with him, and after an enjoyable morning meeting all their friends in Bath he delights in getting away from them all as he and Sophy tuck themselves up snug in their lodgings together.

Mrs Croft has a strong personality. In company at Uppercross she 'calls him to order'; she assists in steering the gig - and how many men would submit to that sort of interference?? But the Admiral is in no way diminished by all this, and there's no sense that he's henpecked. He's just perfectly comfortable with an unusually equal relationship in which he and his wife are a great team. When they are thinking of renting Kellynch he just wants to get into a comfortable home as soon as possible, and treats the business with 'open, trusting liberality', 'not likely to make the smallest difficulty about terms, .....knew he must pay for his convenience; knew what rent a ready-furnished house of that consequence might fetch; should not have been surprised if Sir Walter had asked more.' Meanwhile 'shrewd' Mrs Croft 'asked more questions about the house, and terms, and taxes, than the Admiral himself, and seemed more conversant with business.' (3,5,10)

What are we to make of the Croft's very brief courtship, and the Admiral's apparent view that nothing mattered except that she was pretty? I suspect with his tolerant, easy-going good nature he would probably gave made a reasonable job of marriage with almost any attractive young woman (except either of Anne's sisters, who are beyond amendment). But I think that his line:- 'I had heard of you as a very pretty girl, and what were we to wait for besides?' is a bit of a smokescreen. His inability to distinguish between the Miss Musgroves is more a reflection of their ordinariness than of his deficent perception (Good point - thanks Ramya). I feel sure that when he met Sophy he would have noticed at once that she was not at all ordinary, and that her personality was just as much an attraction for him as her very pretty looks.