BA Amex and Amex Gold both have decent sign up bonuses and (the not available to new customers) BMI Amex/Visa pair are what I'm using - but I'm chasing Avios exclusively (the BA and Iberia loyalty scheme). It depends on what you want back, cash, airline miles etc, how much you'll be putting on the card, and whether you're prepared to pay an annual fee. (The BA Amex gives you a rather useful 241 voucher on an annual spend of 10K and an annual fee of £150).
If you're already a member of certain loyalty schemes, it's possibly worth looking at cards tied into them as a means of boosting your balance, such as the Nectar Amex card, IHG rewards Visa, Hilton Hhonours Visa, Tesco as mentioned above etc.
For way too much info, mainly biased towards airline miles and hotel points, have a look at Headforpoints
EDIT - Headforpoints is nothing to do with me! (Though I'm happy to refer for BA Amex and Amex Gold - though Quidco is normally available for these).