As a firewall, is ISA server more strong than the others?

ISA2006 is a many product in one , in a single software package you ger 

1. A network layer firewall

2. An application layer inspection security gateway

3. Forward and reverse Web proxy and caching server

3. Remote access VPN server

4. Site to site VPN gateway

Its base on which product we want to use software or Hardware Firewall , after getting demo from each product as per our need in our organization. 

A firewall is a powerfull tool to control for trafic and security. but it only depends on the basis of configration if ruleset are strog it should be strong if rulset are weak then you may ask again which one is best. etc

Firewall is broadterm and if you include Hardware Firewalls then these are always superior than software Firewall like ISA Server but perform only filtering and NAT function whereas Software Firewall like ISA Server perform many other functions other than filtering and NAT like publishing, proxy etc. However, if someone budget do allow then obviously Checkpoint and CISCO ASA are market leader in Hardware Firewall and ideally these H/W Firewalls must be used in conjuction with software Firewall.